Queue "process request body" tasks during request body transmission

With this change the "transmit body" algorithm queues "process request body" tasks so that the client can notice the body transmission progress.

Fixes #495.

fetch.bs

@@ -1179,36 +1179,42 @@ or "<code>worker</code>".
  <li><p>If <var>body</var> is null, then <a>queue a fetch task</a> on <var>request</var> to
  <a>process request end-of-body</a> for <var>request</var> and abort these steps.
 
+ <li><p>Let <var>read</var> be the result of <a lt="read a chunk" for=ReadableStream>reading a
+ chunk</a> from <var>body</var>'s <a for=body>stream</a>.
+
  <li>
-  <p>Let <var>read</var> be the result of <a lt="read a chunk" for=ReadableStream>reading a
-  chunk</a> from <var>body</var>'s <a for=body>stream</a>.
+  <p><a>In parallel</a>, while true:
+
+  <ol>
+   <li><p>Wait for <var>read</var> to be fulfilled or rejected.
 
-  <ul>
    <li>
-    <p>When <var>read</var> is fulfilled with an object whose <code>done</code>
-    property is false and whose <code>value</code> property is a
-    <code>Uint8Array</code> object, run these substeps:
+    <p>If <var>read</var> is fulfilled with an object whose <code>done</code> property is false
+    and whose <code>value</code> property is a <code>Uint8Array</code> object, then run these steps:
 
     <ol>
-     <li><p>Let <var>bytes</var> be the <a>byte sequence</a> represented by the
-     <code>Uint8Array</code> object.
+     <li><p>Let <var>bs</var> be the <a>byte sequence</a> represented by the <code>Uint8Array</code>
+     object.
 
-     <li><p>Transmit <var>bytes</var>.
+     <li>
+      <p>Transmit <var>bs</var>. Whenever one or more bytes are transmitted, increase
+      <var>body</var>'s <a for=body>transmitted bytes</a> by the number of transmitted bytes and
+      <a>queue a fetch task</a> on <var>request</var> to <a>process request body</a>
+      for <var>request</var>.
 
-     <li><p>Increase <var>body</var>'s <a for=body>transmitted bytes</a> by <var>bytes</var>'s
-     length.
+      <p class="note no-backref">This step blocks until <var>bs</var> is fully transmitted.
 
-     <li><p>Run the above step again.
+     <li><p>Set <var>read</var> to the result of <a lt="read a chunk" for=ReadableStream>reading a
+     chunk</a> from <var>body</var>'s <a for=body>stream</a>.
     </ol>
 
-   <li><p>When <var>read</var> is fulfilled with an object whose <code>done</code>
-   property is true, <a>queue a fetch task</a> on <var>request</var> to
-   <a>process request end-of-body</a> for <var>request</var>.
+   <li><p>Otherwise, if <var>read</var> is fulfilled with an object whose <code>done</code> property
+   is true, then <a>queue a fetch task</a> on <var>request</var> to
+   <a>process request end-of-body</a> for <var>request</var> and abort these steps.
 
-   <li><p>When <var>read</var> is fulfilled with a value that matches with neither of the
-   above patterns, or <var>read</var> is rejected, <a lt=terminated for=fetch>terminate</a> the
-   ongoing fetch with reason <i>fatal</i>.
-  </ul>
+   <li><p>Otherwise, <a lt=terminated for=fetch>terminate</a> the ongoing fetch with reason
+   <i>fatal</i> and abort these steps.
+  </ol>
 </ol>