"Get" a structured header from a header list.

As discussed in #930 and WICG/cross-origin-embedder-policy#2, this patch adds a "get"
method on header lists that allows consistent extraction of
structured headers.

fetch.bs

@@ -18,6 +18,7 @@ url:https://tools.ietf.org/html/rfc7230#section-3.1.2;text:reason-phrase;type:df
 url:https://tools.ietf.org/html/rfc7234#section-1.2.1;text:delta-seconds;type:dfn;spec:http-caching
 url:https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-2;text:structured header value;type:dfn;spec:header-structure
 url:https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-4.1;text:serializing structured headers;type:dfn;spec:header-structure
+url:https://tools.ietf.org/html/draft-ietf-httpbis-header-structure#section-4.2;text:parsing structured headers;type:dfn;spec:header-structure
 </pre>
 
 <pre class=biblio>
@@ -584,10 +585,32 @@ pair in a <a for=/>header list</a> <var>list</var>, run these steps:
  <li><p><a for="header list">Set</a> <var>name</var>/<var>serializedValue</var> in <var>list</var>.
 </ol>
 
-<p class="note"><a>Structured header values</a> are defined as objects which HTTP can (eventually)
-serialize in interesting and efficient ways. For the moment, Fetch only supports setting these
-objects in <a for=/>header lists</a> by serializing them. In the future the fact that they are
-objects might be preserved end-to-end. [[!HEADER-STRUCTURE]]
+<p>To
+<dfn export for="header list" id=concept-header-list-get-structured-header>get a structured header</dfn>
+given a <var>name</var> and a <var>type</var> from a <a for=/>header list</a> <var>list</var>, run
+these steps:
+
+<ol>
+ <li><p>If <var>type</var> is not one of "<code>dictionary</code>", "<code>list</code>", or
+ "<code>item</code>", return failure.
+ <li><p>Let <var>value</var> be the result of <a for="header list">getting</a> <var>name</var> from
+ <var>list</var>.
+ <li><p>If <var>value</var> is null, return null.
+ <li><p>Let <var>result</var> be the result of executing the <a>parsing structured headers</a>
+ algorithm with <var ignore>input_string</var> set to <var>value</var>, and
+ <var ignore>header_type</var> set to <var>type</var>.
+ <li><p>If parsing failed, return failure.
+ <li><p>Return <var>result</var>.
+</ol>
+
+<p class=note><a>Structured header values</a> are defined as objects which HTTP can (eventually)
+serialize in interesting and efficient ways. For the moment, Fetch only supports <a for=/>header</a>
+<a for=header>values</a> as <a for=/>byte sequences</a>, setting these objects in <a for=/>header
+lists</a> by serializing them, and supports getting these objects from <a for=/>header lists</a> by
+parsing them. In the future the fact that they are objects might be preserved end-to-end.
+[[!HEADER-STRUCTURE]]
+
+</div>
 
 <p>To <dfn export for="header list" id=concept-header-list-combine>combine</dfn> a
 <a for=header>name</a>/<a for=header>value</a> <var>name</var>/<var>value</var> pair in a