Enforce limit on inflight keepalive bytes

Requests with keepalive flag set are allowed to outlive the environment settings object. We want to make sure that such requests do not negatively impact the user experience when a page is unloaded, etc.

This limits the amount of (body) bytes that can be inflight at any point when the request has the keepalive flag set; this flag is also set by sendBeacon().

Background: w3c/beacon#39.

Tests: web-platform-tests/wpt#4878.

fetch.bs

@@ -3179,13 +3179,36 @@ steps:
  <a for=request>header list</a>.
 
  <li>
-  <p>If <var>contentLengthValue</var> is non-null, <var>httpRequest</var>'s
-  <a>keepalive flag</a> is set, and <var>contentLengthValue</var> is greater than a
-  user-agent-defined maximum, then return a <a>network error</a>.
+  <p>If <var>contentLengthValue</var> is non-null and <var>httpRequest</var>'s
+  <a for=request>keepalive flag</a> is set, then:
 
-  <p class="note no-backref">The above user-agent-defined maximum ensures that requests that are
-  allowed to outlive the <a>environment settings object</a> and contain
-  a body, have a bounded size and are not allowed to stay alive indefinitely.
+  <ol>
+   <li><p>Let <var>inflightKeepaliveBytes</var> be zero.
+
+   <li><p>Let <var>group</var> be <var>httpRequest</var>'s <a>client</a>'s <a>fetch group</a>.
+
+   <li><p>Let <var>inflightRecords</var> be the set of <a for="fetch group">fetch records</a> in
+   <var>group</var> whose <a for="fetch record">request</a> has its
+   <a for=request>keepalive flag</a> set and <a>done flag</a> unset.
+
+   <li>
+    <p><a for=set>For each</a> <var>fetchRecord</var> in <var>inflightRecords</var>:
+
+    <ol>
+     <li><p>Let <var>inflightRequest</var> be <var>fetchRecord</var>'s
+     <a for="fetch record">request</a>.
+
+     <li><p>Increment <var>inflightKeepaliveBytes</var> by <var>inflightRequest</var>'s
+     <a for=request>body</a>'s <a for=body>total bytes</a>.
+    </ol>
+
+   <li><p>If the sum of <var>contentLengthValue</var> and <var>inflightKeepaliveBytes</var> is
+   greater than 64 kibibytes, then return a <a>network error</a>.
+  </ol>
+
+  <p class="note no-backref">The above limit ensures that requests that are allowed to outlive the
+  <a>environment settings object</a> and contain a body, have a bounded size and are not allowed to
+  stay alive indefinitely.
 
  <li><p>If <var>httpRequest</var>'s <a for=request>referrer</a> is a <a for=/>URL</a>, then
  <a for="header list">append</a> `<code>Referer</code>`/<var>httpRequest</var>'s
@@ -4158,7 +4181,7 @@ run these steps:
 
 <ol>
  <li>
-  <p>If <var>object</var> is a <a>sequence</a>, then for each <var>header</var> in
+  <p>If <var>object</var> is a <a>sequence</a>, then <a for=list>for each</a> <var>header</var> in
   <var>object</var>, run these substeps:
 
   <ol>
@@ -4170,8 +4193,8 @@ run these steps:
    <var>headers</var>. Rethrow any exception.
   </ol>
 
- <li><p>Otherwise, <var>object</var> is a <a>record</a>, then for each <a for=record>mapping</a>
- (<var>key</var>, <var>value</var>) in <var>object</var>, <a lt=append for=Headers>append</a>
+ <li><p>Otherwise, <var>object</var> is a <a>record</a>, then <a for=map>for each</a> <var>key</var>
+ → <var>value</var> in <var>object</var>, <a lt=append for=Headers>append</a>
  <var>key</var>/<var>value</var> to <var>headers</var>. Rethrow any exception.
 </ol>