Use extract a length from Fetch

The only information browsers use for progress events for responses is the Content-Length header.

Align on that and only use body's length concept for uploads.

Tests: web-platform-tests/wpt#27837.

Fetch PRs: whatwg/fetch#1183 & whatwg/fetch#1184.

xhr.bs

@@ -783,16 +783,20 @@ return <a>this</a>'s <a>cross-origin credentials</a>.
  <li><p><a>Fire a progress event</a> named <a event><code>loadstart</code></a> at <a>this</a>
  with 0 and 0.
 
+ <li><p>Let <var>uploadedBytesLength</var> be 0.
+
+ <li><p>Let <var>totalUploadedBytesLength</var> be <var>req</var>'s <a for=request>body</a>'s
+ <a for=body>length</a>, if <var>req</var>'s <a for=request>body</a>'s <a for=body>length</a> is
+ non-null; otherwise 0.
+
  <li><p>If <a>this</a>'s <a>upload complete flag</a> is unset and <a>this</a>'s
  <a>upload listener flag</a> is set, then <a>fire a progress event</a> named
- <a event><code>loadstart</code></a> at <a>this</a>'s <a>upload object</a> with 0 and
- <var>req</var>'s <a for=request>body</a>'s <a>total bytes</a>.
+ <a event><code>loadstart</code></a> at <a>this</a>'s <a>upload object</a> with
+ <var>uploadedBytesLength</var> and <var>totalUploadedBytesLength</var>.
 
  <li><p>If <a>this</a>'s <a>state</a> is not <i>opened</i> or <a>this</a>'s
  <a><code>send()</code> flag</a> is unset, then return.
 
- <li><p>Let <var>uploadedBytesLength</var> be 0.
-
  <li>
  <p>Let <var>processRequestBody</var>, given a <var>bytesLength</var>, be these steps:
 
@@ -803,8 +807,7 @@ return <a>this</a>'s <a>cross-origin credentials</a>.
 
  <li><p>If <a>this</a>'s <a>upload listener flag</a> is set, then <a>fire a progress event</a>
  named <a event><code>progress</code></a> at <a>this</a>'s <a>upload object</a> with
- <var>uploadedBytesLength</var> and <var>req</var>'s <a for=request>body</a>'s
- <a>total bytes</a>.
+ <var>uploadedBytesLength</var> and <var>totalUploadedBytesLength</var>.
  <!-- upload complete flag can never be set here I hope -->
  </ol>
 
@@ -818,18 +821,17 @@ return <a>this</a>'s <a>cross-origin credentials</a>.
 
  <li><p>If <a>this</a>'s <a>upload listener flag</a> is unset, then return.
 
- <li><p>Let <var>length</var> be <var>req</var>'s
- <a for=request>body</a>'s
- <a>total bytes</a>.
-
  <li><p><a>Fire a progress event</a> named <a event><code>progress</code></a> at <a>this</a>'s
- <a>upload object</a> with <var>uploadedBytesLength</var> and <var>length</var>.
+ <a>upload object</a> with <var>uploadedBytesLength</var> and
+ <var>totalUploadedBytesLength</var>.
 
  <li><p><a>Fire a progress event</a> named <a event><code>load</code></a> at <a>this</a>'s
- <a>upload object</a> with <var>uploadedBytesLength</var> and <var>length</var>.
+ <a>upload object</a> with <var>uploadedBytesLength</var> and
+ <var>totalUploadedBytesLength</var>.
 
  <li><p><a>Fire a progress event</a> named <a event><code>loadend</code></a> at <a>this</a>'s
- <a>upload object</a> with <var>uploadedBytesLength</var> and <var>length</var>.
+ <a>upload object</a> with <var>uploadedBytesLength</var> and
+ <var>totalUploadedBytesLength</var>.
  </ol>
  <!-- upload complete flag can never be set here I hope -->
 
@@ -853,6 +855,11 @@ return <a>this</a>'s <a>cross-origin credentials</a>.
  <li><p>If <a>this</a>'s <a for=XMLHttpRequest>response</a>'s <a for=response>body</a> is null,
  then run <a>handle response end-of-body</a> for <a>this</a> and return.
 
+ <li><p>Let <var>length</var> be the result of <a for="header list">extracting a length</a> from
+ <a>this</a>'s <a for=XMLHttpRequest>response</a>'s <a for=response>header list</a>.
+
+ <li><p>If <var>length</var> is not an integer, then set it to 0.
+
  <li>
  <p>Let <var>processBodyChunk</var> given <var>bytes</var> be these steps:
 
@@ -872,8 +879,7 @@ return <a>this</a>'s <a>cross-origin credentials</a>.
 
  <li><p><a>Fire a progress event</a> named <a event><code>progress</code></a> at <a>this</a>
  with <a>this</a>'s <a>received bytes</a>'s <a for="byte sequence">length</a> and
- <a>this</a>'s <a for=XMLHttpRequest>response</a>'s <a for=response>body</a>'s
- <a>total bytes</a>.
+ <var>length</var>.
  </ol>
 
  <li><p>Let <var>processEndOfBody</var> be this step: run <a>handle response end-of-body</a> for
@@ -968,8 +974,10 @@ return <a>this</a>'s <a>cross-origin credentials</a>.
  <li><p>Let <var>transmitted</var> be <var>xhr</var>'s <a>received bytes</a>'s
  <a for="byte sequence">length</a>.
 
- <li><p>Let <var>length</var> be <var>xhr</var>'s <a for=XMLHttpRequest>response</a>'s
- <a for=response>body</a>'s <a for=body>total bytes</a>.
+ <li><p>Let <var>length</var> be the result of <a for="header list">extracting a length</a> from
+ <a>this</a>'s <a for=XMLHttpRequest>response</a>'s <a for=response>header list</a>.
+
+ <li><p>If <var>length</var> is not an integer, then set it to 0.
 
  <li><p>If <var>xhr</var>'s <a>synchronous flag</a> is unset, then <a>fire a progress event</a>
  named <a event><code>progress</code></a> at <var>xhr</var> with <var>transmitted</var> and