Add timing information about interim responses

See w3c/resource-timing#345 for context.

fetch.bs

@@ -343,6 +343,7 @@ following <a for=struct>items</a>: [[RESOURCE-TIMING]] [[NAVIGATION-TIMING]]
  <dt><dfn export for="fetch timing info">post-redirect start time</dfn> (default 0)
  <dt><dfn export for="fetch timing info">final service worker start time</dfn> (default 0)
  <dt><dfn export for="fetch timing info">final network-request start time</dfn> (default 0)
+ <dt><dfn export for="fetch timing info">first interim network-response start time</dfn> (default 0)
  <dt><dfn export for="fetch timing info">final network-response start time</dfn> (default 0)
  <dt><dfn export for="fetch timing info">end time</dfn> (default 0)
  <dd>A {{DOMHighResTimeStamp}}.
@@ -5995,9 +5996,8 @@ optional boolean <var>forceNewConnection</var> (default false), run these steps:
       <p>While true:
 
       <ol>
-       <li><p>If <var>timingInfo</var>'s
-       <a for="fetch timing info">final network-response start time</a> is 0, then set
-       <var>timingInfo</var>'s <a for="fetch timing info">final network-response start time</a> to
+       <li><p>Set <var>timingInfo</var>'s
+       <a for="fetch timing info">final network-response start time</a> to the
        <a for=/>coarsened shared current time</a> given <var>fetchParams</var>'s
        <a for="fetch params">cross-origin isolated capability</a>, immediately after the user
        agent's HTTP parser receives the first byte of the response (e.g., frame header bytes for
@@ -6015,6 +6015,12 @@ optional boolean <var>forceNewConnection</var> (default false), run these steps:
         processing we now perform here inline. -->
 
         <ol>
+         <li><p>If <var>timingInfo</var>'s
+         <a for="fetch timing info">first interim network-response start time</a> is 0, then set
+         <var>timingInfo</var>'s
+         <a for="fetch timing info">first interim network-response start time</a> to
+         <var>timingInfo</var>'s <a for="fetch timing info">final network-response start time</a>.
+
          <li><p>If <var>request</var>'s <a for=request>mode</a> is "<code>websocket</code>" and
          <var>status</var> is 101, then <a for=iteration>break</a>.