Add Server-Timing header infrastructure

For w3c/server-timing#87.

(This does not deal with support in trailer fields, that's tracked by w3c/server-timing#68.)

fetch.bs

@@ -273,6 +273,9 @@ following <a for=struct>items</a>: [[RESOURCE-TIMING]] [[NAVIGATION-TIMING]]
 
  <dt><dfn export for="fetch timing info">final connection timing info</dfn> (default null)
  <dd>Null or a <a for=/>connection timing info</a>.
+
+ <dt><dfn export for="fetch timing info">server-timing headers</dfn> (default « »)
+ <dd>A <a for=/>list</a> of strings.
 </dl>
 
 <p>To
@@ -4341,7 +4344,16 @@ steps:
    <a>creating an opaque timing info</a> for <var>fetchParams</var>'s
    <a for="fetch params">timing info</a>.</p></li>
   </ol>
- </li>
+
+ <li>
+  <p>Otherwise, if <var>fetchParams</var>'s <a for="fetch params">request</a>'s
+  <a for=request>client</a> is a <a>secure context</a>, set <var>response</var>'s
+  <a for=response>timing info</a>'s <a for="fetch timing info">server-timing headers</a> to the
+  result of <a for="header list">getting, decoding, and splitting</a> `<code>Server-Timing</code>`
+  from <var>response</var>'s <a for=response>header list</a>.
+
+  <p>The user agent may decide to expose `<code>Server-Timing</code>` headers to non-secure contexts
+  requests as well.
 
  <li>
   <p>Let <var>processResponseEndOfBody</var> be the following steps: