Work-in-progress: Refine srcObject's MediaSourceHandle behavior

Adds text intending to help prevent using one or more
`MediaSourceHandle` for the same underlying `MediaSource` for
simultaneous, concurrent attachments to media elements:

* Updates `MediaSource.getHandle()` to succeed at most once per
  `MediaSource` instance.
* Updates `MediaSourceHandle` transfer section to more clearly indicate
  that transferring a handle out of a context makes that handle
  unavailable for transferring again out of the same context (unless it
  is first transferred back to that context).
* Updates the "attaching to a media element" text to indicate that a
  `MediaSourceHandle` can be attached successfully to at most one media
  element at a time, and with the previous 2 clarifications, ensures
  that the underlying worker context `MediaSource` can only be attached
  successfully to at most one media element at a time.

Note that legacy MSE object URL attachments similarly already achieve
the same intent in existing implementations. The switch to more clear
`srcObject` usage for worker MSE attachment affords the spec an
opportunity to be more clear about this intent.

This refinement originates from discussion on the previous PR with
@karlt: w3c#305

media-source-respec.html

@@ -283,6 +283,12 @@ <h2><dfn>MediaSource</dfn> Object</h2>
         <a href="#htmlmediaelement-extensions">HTMLMediaElement Extensions</a> to modify
         {{HTMLMediaElement}}.{{HTMLMediaElement/seekable}} behavior.</p>
 
+      <p>Each {{MediaSource}} object has a <dfn data-dfn-for="MediaSource">[[\handle already retrieved]]</dfn> internal
+        slot that is a boolean that keeps track of whether {{MediaSource/getHandle()}} has already created and returned
+        a {{MediaSourceHandle}} object for the {{MediaSource}} object. It is initialized to false when the
+        {{MediaSource}} object is created, and is consulted and updated by {{MediaSource/getHandle()}} to prevent
+        creating multiple distinct {{MediaSourceHandle}} objects for by same {{MediaSource}} object.</p>
+
       <div><pre class="idl">enum ReadyState {
     "closed",
     "open",
@@ -650,7 +656,19 @@ <h3>Attributes</h3>
             <p class="issue">Consider requiring conforming implementations to let such handle be created on a {{Window}}
               {{MediaSource}} object, too?</p>
           </li>
-          <li>Create a new {{MediaSourceHandle}} object and associated resources.</li>
+          <li>If the {{MediaSource/readyState}} attribute is not {{ReadyState/""closed""}} then throw an
+            {{InvalidStateError}} exception and abort these steps.</li>
+          <li>If {{MediaSource/[[handle already retrieved]]}} is true, then throw an {{InvalidStateError}} exception and
+            abort these steps.</li>
+          <li>Create a new {{MediaSourceHandle}} object and associated resources, and link it internally to this
+            {{MediaSource}}.</li>
+          <li>Set {{MediaSource/[[handle already retrieved]]}} to be true.
+            <p class="note">This will prevent creation of multiple {{MediaSourceHandle}} objects for this
+              {{MediaSource}} object, to help prevent this {{MediaSource}} from being attached using distinct handles
+              simultaneously to distinct media elements. Legacy object URL attachments allow distinct object URLs for
+              the same {{MediaSource}} object, though simultaneous attachment of the same {{MediaSource}} via those
+              distinct object URLs to distinct media elements should also succeed for at most one media element.</p>
+          </li>
           <li>Return the new object.</li>
         </ol>
         <div><em>No parameters.</em></div><div><em>Return type: </em>{{MediaSourceHandle}}</div></dd>
@@ -770,6 +788,7 @@ <h4><dfn>Attaching to a media element</dfn></h4>
               object mentioned there, and the {{MediaSourceHandle}} object is the media provider object.
               <p class="issue">MediaSourceHandle needs to be added to HTMLMediaElement's MediaProvider IDL typedef and
               related text involving media provider objects.</p>
+              BIG TODO: add text around preventing using the handle simultaneously for for than one media element.
             </li>
           </ul>
 
@@ -1169,6 +1188,12 @@ <h2><dfn>MediaSourceHandle</dfn> Object</h2>
         maintain an implicit internal slot referencing the underlying {{MediaSource}} to enable [=attaching to a media
         element=] using {{HTMLMediaElement/srcObject}} and consequent setup of an attachment's [=cross-context
         communication model=].
+
+        Once transferred out of a context, the handle MUST no longer be usable in that context.
+        <p class="note">For example, once a worker transfers a {{MediaSourceHandle}} to another context, it cannot
+        transfer that handle again unless the another context first transfers it back to the worker. This prevents
+        ability to clone {{MediaSourceHandle}} objects and helps prevent simultaneous attachment to distinct media
+        elements using handles to the same underlying {{MediaSource}}.</p>
       </section>
     </section>