Define Content-Length parser

As the body concept is refactored for #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 #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>