Define Content-Length parser

As the body concept is refactored for whatwg#604, XMLHttpRequest will need to use this algorithm for its events. We also want to require browsers to use this algorithm instead of the one defined by HTTP as part of whatwg#1156.

Tests: web-platform-tests/wpt#10548.

fetch.bs

@@ -2807,10 +2807,44 @@ consideration for the security consequences. New exceptions can be proposed by
 <a href=https://github.com/whatwg/fetch/issues/new>filing an issue</a>.
 
 
+<h3 id=content-length-header>`<code>Content-Length</code>` header</h3>
+
+<p>The `<code>Content-Length</code>` header is largely defined in HTTP. Its processing model is
+defined here as the model defined in HTTP is not compatible with web content. [[HTTP]]
+
+<p>To <dfn export for="header list" lt="extract a length|extracting a length">extract a length</dfn>
+from a <a for=/>header list</a> <var>headers</var>, run these steps:
+
+<ol>
+ <li><p>Let <var>values</var> be the result of
+ <a for="header list">getting, decoding, and splitting</a> `<code>Content-Length</code>` from
+ <var>headers</var>.
+
+ <li><p>If <var>values</var> is null, then return null.
+
+ <li><p>Let <var>candidateValue</var> be null.
+
+ <li>
+  <p><a for=list>For each</a> <var>value</var> of <var>values</var>:
+
+  <ol>
+   <li><p>If <var>candidateValue</var> is null, then set <var>candidateValue</var> to
+   <var>value</var>.
+
+   <li><p>Otherwise, if <var>value</var> is not <var>candidateValue</var>, return failure.
+  </ol>
+
+ <li><p>If <var>candidateValue</var> is the empty string or has a <a for=/>code point</a> that is
+ not an <a for=/>ASCII digit</a>, then return null.
+
+ <li><p>Return <var>candidateValue</var>, interpreted as decimal number.
+</ol>
+
+
 <h3 id=content-type-header>`<code>Content-Type</code>` header</h3>
 
 <p>The `<code>Content-Type</code>` header is largely defined in HTTP. Its processing model is
-defined here as the ABNF defined in HTTP is not compatible with web content. [[HTTP]]
+defined here as the model defined in HTTP is not compatible with web content. [[HTTP]]
 
 <p>To
 <dfn export for="header list" lt="extract a MIME type|extracting a MIME type" id=concept-header-extract-mime-type>extract a MIME type</dfn>