Add more timing iformation about (interim) responses

See w3c/resource-timing#345

Since early hints have landed, there are additional useful timestamps
that are currently not exposed:

- First interim response start (e.g. when we received a 103)
  as a different timestamp from the final response start
  (e.g. when we received the 200)

- Final headers received (when the last header has been received and
  we're ready for the body)

- First bytes of the body received

The naming in whatwg#345 is not finalized yet, but it's clear that these
are the 3 interesting timestamps. This PR exposes those for later
use by resource timing.

fetch.bs

@@ -280,7 +280,10 @@ 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">final network-response headers end time</dfn> (default 0)
+ <dt><dfn export for="fetch timing info">response body start time</dfn> (default 0)
  <dt><dfn export for="fetch timing info">end time</dfn> (default 0)
  <dd>A {{DOMHighResTimeStamp}}.
 
@@ -5528,22 +5531,32 @@ 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
        HTTP/2 or response status line for HTTP/1.x).
 
        <li><p>Wait until all the HTTP response headers are transmitted.
 
+       <li><p>Set <var>timingInfo</var>'s
+       <a for="fetch timing info">final network-response headers end time</a> to
+       <a for=/>coarsened shared current time</a> given <var>fetchParams</var>'s
+       <a for="fetch params">cross-origin isolated capability</a>.
+
        <li><p>Let <var>status</var> be the HTTP response's status code.
 
        <li>
         <p>If <var>status</var> is in the range 100 to 199, inclusive:
 
         <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>status</var> is 101, <a for=iteration>break</a>.
 
          <li><p>If <var>status</var> is 103 and <var>fetchParams</var>'s
@@ -5723,6 +5736,13 @@ optional boolean <var>forceNewConnection</var> (default false), run these steps:
         <p>If one or more bytes have been transmitted from <var>response</var>'s message body, then:
 
         <ol>
+         <li><p>If <var>fetchParams</var>'s <a for="fetch params">timing info</a>'s
+         <a for="fetch timing info">response body start time</a> is 0, then set
+         <var>fetchParams</var>'s <a for="fetch params">timing info</a>'s
+         <a for="fetch timing info">response body 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>.
+
          <li><p>Let <var>bytes</var> be the transmitted bytes.
 
          <li><p>Let <var>codings</var> be the result of <a>extracting header list values</a> given