Fix toBlob() and convertToBlob()

In particular:

* Neither method dealt properly with "serialization of a the bitmap as a file" returning null.
* Neither method clearly indicated what had to happen synchronously (bitmap copy) and asynchronously (converting to a file).
* toBlob() did not indicate the Realm of the new Blob object.
* toBlob() invoked IDL's callback concept incorrectly.
* convertToBlob() was weird with in parallel and queue a task.
* convertToBlob() did not list a task source.

Fixes #3386.

Author: annevk

source

@@ -59599,31 +59599,43 @@ callback <dfn>BlobCallback</dfn> = void (<span>Blob</span>? blob);</pre>
   <var>quality</var>)</code></dfn> method, when invoked, must run these steps:</p>
 
   <ol>
-
-<!--ADD-TOPIC:Security-->
    <li><p>If this <code>canvas</code> element's bitmap's <span
    data-x="concept-canvas-origin-clean">origin-clean</span> flag is set to false, then throw a
-   <span>"<code>SecurityError</code>"</span> <code>DOMException</code>.</p>
-<!--REMOVE-TOPIC:Security-->
+   <span>"<code>SecurityError</code>"</span> <code>DOMException</code>.</p></li>
+
+   <li><p>Let <var>result</var> be null.</p></li>
+
+   <li><p>If this <code>canvas</code> element's bitmap has pixels (i.e., neither its horizontal
+   dimension nor its vertical dimension is zero), then set <var>result</var> to a copy of this
+   <code>canvas</code> element's bitmap.
 
    <li>
-    <p>If this <code>canvas</code> element's bitmap has no pixels (i.e. either its horizontal
-    dimension or its vertical dimension is zero) then let <var>result</var> be null.</p>
+    <p>Run these steps <span>in parallel</span>:</p>
 
-    <p>Otherwise, let <var>result</var> be a <code>Blob</code> object representing <span data-x="a
-    serialization of the bitmap as a file">a serialization of this <code>canvas</code> element's
-    bitmap as a file</span>, passing <var>type</var> and <var>quality</var> if they were given. <ref
-    spec=FILEAPI></p>
-   </li>
+    <ol>
+     <li><p>If <var>result</var> is non-null, then set <var>result</var> to <span data-x="a
+     serialization of the bitmap as a file">a serialization of <var>result</var> as a file</span>,
+     with <var>type</var> and <var>quality</var> if they were given.</p></li>
 
-   <li><p>Return, but continue running these steps <span>in parallel</span>.</p></li>
+     <li>
+      <p><span>Queue a task</span> to run these steps:</p>
 
-   <li><p><span>Queue a task</span> to <span data-x="es-invoking-callback-functions">invoke</span>
-   the <code>BlobCallback</code> <var>callback</var> with <var>result</var> as its argument. The
-   <span>task source</span> for this task is the
-   <!--en-GB--><dfn id="canvas-blob-serialisation-task-source">canvas blob serialization task
-   source</dfn>.</p></li>
+      <ol>
+       <li><p>If <var>result</var> is non-null, then set <var>result</var> to a new
+       <code>Blob</code> object, created in the <span data-x="concept-relevant-realm">relevant
+       Realm</span> of this <code>canvas</code> element, representing <var>result</var>. <ref
+       spec=FILEAPI></p></li>
 
+       <li><p><span data-x="es-invoking-callback-functions">Invoke</span> <var>callback</var> with
+       « <var>result</var> ».</p></li>
+      </ol>
+
+      <p>The <span>task source</span> for this task is the
+      <!--en-GB--><dfn id="canvas-blob-serialisation-task-source">canvas blob serialization task
+      source</dfn>.</p>
+     </li>
+    </ol>
+   </li>
   </ol>
 
   <p>The <dfn><code
@@ -65286,28 +65298,44 @@ interface <dfn>OffscreenCanvas</dfn> : <span>EventTarget</span> {
    promise rejected with a <span>"<code>SecurityError</code>"</span> <code>DOMException</code>.</p>
 
    <li><p>If this <code>OffscreenCanvas</code> object's <span
-   data-x="offscreencanvas-bitmap">bitmap</span> has no pixels (i.e. either its
+   data-x="offscreencanvas-bitmap">bitmap</span> has no pixels (i.e., either its
    horizontal dimension or its vertical dimension is zero) then return a promise rejected with an
    <span>"<code>IndexSizeError</code>"</span> <code>DOMException</code>.</p></li>
 
+   <li><p>Let <var>bitmap</var> be a copy of this <code>OffscreenCanvas</code> object's <span
+   data-x="offscreencanvas-bitmap">bitmap</span>.</p></li>
+
    <li><p>Let <var>result</var> be a new promise object.</p></li>
 
-   <li><p>Return <var>result</var>, and <span>queue a task</span> to run the remaining steps
-   <span>in parallel</span>.</p></li>
+   <li>
+    <p>Run these steps <span>in parallel</span>:</p>
+
+    <ol>
+     <li><p>Let <var>file</var> be <span data-x="a serialization of the bitmap as a file">a
+     serialization of <var>bitmap</var> as a file</span>, with <var>options</var>'s <dfn><code
+     data-x="image-encode-options-type">type</code></dfn> and <dfn><code
+     data-x="image-encode-options-quality">quality</code></dfn>.</p></li>
+
+     <li>
+      <p><span>Queue a task</span> to run these steps:</p>
 
-   <li><p>Let <var>blob</var> be a <code>Blob</code> object, created in the <span
-   data-x="concept-relevant-realm">relevant Realm</span> of this <code>OffscreenCanvas</code>
-   object, representing <span data-x="a serialization of the bitmap as a file">a serialization of
-   this <code>OffscreenCanvas</code> object's <span data-x="offscreencanvas-bitmap">bitmap</span> as
-   a file</span>, passing the values of the <dfn><code
-   data-x="image-encode-options-type">type</code></dfn> and <dfn><code
-   data-x="image-encode-options-quality">quality</code></dfn> fields of <var>options</var>, if
-   <var>options</var> was specified. <ref spec=FILEAPI></p></li>
+      <ol>
+       <li><p>If <var>file</var> is null, then reject <var>result</var> with an
+       <span>"<code>EncodingError</code>"</span> <code>DOMException</code>.</p></li>
 
-   <li><p>If <var>blob</var> is null, then reject <var>result</var> with an
-   <span>"<code>EncodingError</code>"</span> <code>DOMException</code>.</p></li>
+       <li><p>Otherwise, resolve <var>result</var> with a new <code>Blob</code> object, created in
+       the <span data-x="concept-relevant-realm">relevant Realm</span> of this
+       <code>OffscreenCanvas</code> object, representing <var>file</var>. <ref
+       spec=FILEAPI></p></li>
+      </ol>
 
-   <li><p>Otherwise, resolve <var>result</var> with <var>blob</var>.</p></li>
+      <p>The <span>task source</span> for this task is the <span>canvas blob serialization task
+      source</span>.</p>
+     </li>
+    </ol>
+   </li>
+
+   <li><p>Return <var>result</var>.</p></li>
   </ol>
 
   <p>The <dfn><code