Update type of MediaMetadata's artwork

Fixes #237

Introduce a slot where we store the FrozenArray so that the getter always returns the same object.
Make sure the slot is reset when the setter is called.

index.bs

@@ -50,6 +50,7 @@ table td, table th {
 
 <pre class="link-defaults">
 spec:html; type:element; text:link
+spec:webidl; type:interface; text:object
 </pre>
 
 <pre class="anchors">
@@ -1094,9 +1095,10 @@ dictionary MediaMetadataInit {
 </p>
 
 <p>
-  A {{MediaMetadata}} has an associated list of <dfn for="MediaMetadata">artwork
-  images</dfn>, which is a list of type <a idl>object</a> on the interface
-  but a list of type {{MediaImage}} internally.
+  A {{MediaMetadata}} has an associated sequence of <dfn for="MediaMetadata">artwork
+  images</dfn>, which is a sequence of type {{MediaImage}}.
+  A {{MediaMetadata}} also has has an associated <dfn for="MediaMetadata">
+  converted artwork images</dfn> which is initially <code>undefined</code>.
 </p>
 
 <p>
@@ -1140,10 +1142,9 @@ dictionary MediaMetadataInit {
     </li>
     <li>
       Run the <a>convert artwork algorithm</a> with <var>init</var>'s
-      {{MediaMetadataInit/artwork}} as <var>input</var>,
-      where the <var>input</var> is a list of type {{MediaImage}},
-      and set <var>metadata</var>'s <a for="MediaMetadata">artwork images</a>
-      as the result if it succeeded.
+      {{MediaMetadataInit/artwork}} as <var>input</var> and set
+      <var>metadata</var>'s <a for="MediaMetadata">artwork images</a> as the
+      result if it succeeded.
     </li>
     <li>
       Let <var>chapters</var> be an empty list of type {{ChapterInformation}}.
@@ -1166,7 +1167,7 @@ dictionary MediaMetadataInit {
 </p>
 
 When the <dfn>convert artwork algorithm</dfn> with <var>input</var> parameter is
-invoked, where the <var>input</var> is a list of type {{MediaImage}},
+invoked, where the <var>input</var> is a sequence of type {{MediaImage}},
 the user agent MUST run the following steps:
 <ol>
   <li>
@@ -1230,62 +1231,56 @@ the user agent MUST run the following steps:
 <p>
   The <dfn attribute for="MediaMetadata">artwork</dfn>
   attribute reflects the {{MediaMetadata}}'s <a for="MediaMetadata">artwork
-  images</a>. On getting, it MUST return the result of the following steps:
+  images</a>. On getting, it MUST run the following steps:
   <ol>
     <li>
-      Let <var>frozenArtwork</var> be an empty list of type <a idl>object</a>.
-    </li>
-    <li>
-      For each <var>entry</var> in the {{MediaMetadata}}'s <a
-      for="MediaMetadata">artwork images</a>, perform the following steps:
+      If the {{MediaMetadata}}'s <dfn>converted artwork images</dfn> is <code>undefined</code>,
+      run the following steps:
       <ol>
         <li>
-          Let <var>image</var> be a new {{MediaImage}}.
+          Let <var>frozenArtwork</var> be a Javascript Array value.
         </li>
         <li>
-          Set <var>image</var>'s {{MediaImage/src}} to <var>entry</var>'s
-          {{MediaImage/src}}.
-        </li>
-        <li>
-          Set <var>image</var>'s {{MediaImage/sizes}} to <var>entry</var>'s
-          {{MediaImage/sizes}}.
-        </li>
-        <li>
-          Set <var>image</var>'s {{MediaImage/type}} to <var>entry</var>'s
-          {{MediaImage/type}}.
-        </li>
-        <li>
-          Convert <var>image</var> into an <var>object</var>
-          whose type is <a idl>object</a>.
+          For each <var>entry</var> in the {{MediaMetadata}}'s <a
+          for="MediaMetadata">artwork images</a>, perform the following steps:
+          <ol>
+            <li>
+              Convert <var>entry</var> into a Javascript object named <var>image</var>.
+            </li>
+            <li>
+              Perform [=!=] <a abstract-op>SetIntegrityLevel</a>(<var>image</var>,
+              "<code>frozen</code>"), to prevent accidental mutation by scripts.
+            </li>
+            <li>
+              Push <var>image</var> to <var>frozenArtwork</var>.
+            </li>
+          </ol>
         </li>
-        <!-- XXX IDL dictionaries are usually returned by value, so don't need
-        to be immutable. But FrozenArray reifies the dictionaries to mutable JS
-        objects accessed by reference, so we explicitly freeze them. It would be
-        better to do this with IDL primitives instead of JS - see
-        https://www.w3.org/Bugs/Public/show_bug.cgi?id=29004 -->
         <li>
-          Call {{Object/freeze(O)}} on <var>image</var>, to prevent accidental
-          mutation by scripts.
+          Perform [=!=] <a abstract-op>SetIntegrityLevel</a>(<var>frozenArtwork</var>, "<code>frozen</code>").
         </li>
         <li>
-          Append the <var>object</var> to <var>frozenArtwork</var>.
+          Set the {{MediaMetadata}}'s <a>converted artwork images</a> to <var>frozenArtwork</var>.
         </li>
       </ol>
     </li>
     <li>
-      <a>Create a frozen array</a> from <var>frozenArtwork</var>.
+      Return the {{MediaMetadata}}'s <a>converted artwork images</a>.
     </li>
   </ol>
   On setting, it MUST run the following steps:
   <ol>
     <li>
-      Convert the <var>frozenArtwork</var> from a list of type <a idl>object</a>
-      into a list of type {{MediaImage}}.
+      Let <var>convertedArtwork</var> be the result of converting
+      <var>frozenArtwork</var> into a sequence of type {{MediaImage}}.
     </li>
     <li>
-      Run <a>convert artwork algorithm</a> with the converted list as
+      Run <a>convert artwork algorithm</a> with the converted sequence as
       <var>input</var>, and set the {{MediaMetadata}}'s
-      <a for="MediaMetadata">artwork images</a> as the result if it succeeded.
+      <a for="MediaMetadata">artwork images</a> as the result if it succeeds.
+    </li>
+    <li>
+      Set the {{MediaMetadata}}'s <a>converted artwork images</a> to <code>undefined</code>.
     </li>
   </ol>
 </p>
@@ -1384,11 +1379,12 @@ dictionary ChapterInformationInit {
     </li>
     <li>
       Let {{ChapterInformationInit/artwork}} be the result of running the
-      <a>convert artwork algorithm</a>.
+      <a>convert artwork algorithm</a> with <var>init</var>'s
+      {{ChapterInformation/artwork}} as <var>input</var>.
     </li>
     <li>
       Set <var>chapterInfo</var>'s <a for="ChapterInformation">artwork
-      images</a> to the result of [=Create a frozen array|creating a frozen
+      images</a> be the result of [=Create a frozen array|creating a frozen
       array=] from {{ChapterInformationInit/artwork}}.
     </li>
     <li>