Update type of MediaMetadata's artwork

Fixes w3c#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">
@@ -1230,54 +1231,66 @@ 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}}.
-        </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>.
+          Let <var>frozenArtwork</var> be an empty list of type <a idl>object</a>.
         </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.
+          For each <var>entry</var> in the {{MediaMetadata}}'s <a
+          for="MediaMetadata">artwork images</a>, perform the following steps:
+          <ol>
+            <li>
+              Let <var>image</var> be a new {{MediaImage}}.
+            </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>.
+            </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.
+            </li>
+            <li>
+              Append the <var>object</var> to <var>frozenArtwork</var>.
+            </li>
+          </ol>
         </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>
+      Set the {{MediaMetadata}}'s <a>converted artwork images</a> to <code>undefined</code>.
+    </li>
     <li>
       Convert the <var>frozenArtwork</var> from a list of type <a idl>object</a>
       into a list of type {{MediaImage}}.