Refine srcObject's MediaSourceHandle behavior

Overall intent for a `MediaSourceHandle` is that it should be
[Transferable], and be restricted to "at most one" successful attachment
to the underlying `MediaSource` by a handle. Further, if a `Window`
`MediaSource` was previously attached using a legacy MSE object URL,
subsequent attempts to use that `MediaSource` instance's
`MediaSourceHandle` for attachment must also fail.

* Changes `MediaSource.getHandle()` to be a `[SameObject] readonly
  attribute` named `handle`.

* Adds HTMLMediaElement.srcObject extension subsection.

* Updates MediaSourceHandle transfer and related attachment text to more
  clearly indicate that transferring a handle prevents re-transfer of
  that instance due to the [[Detached]] internal slot's logic, and that
  transfer is also synchronously prevented if the handle instance's
  [[has ever been assigned as srcobject]] internal slot is true. Also
  adds multiple notes to describe intent and to clarify.

* Updates the "attaching to a media element" text to indicate that a
  `MediaSourceHandle` can be attached successfully to at most one media
  element ever.
  Note that legacy MSE object URL attachments similarly already achieve
  the same intent in existing implementations. This change includes a
  clarification for MSE object URL attachments, since it was never the
  intent even for main-thread `MediaSource` attachments for them to be
  successfully attached to more than one `HTMLMediaElement` at a time.
  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) and from lengthy
discussion on this change itself at
w3c#306.

This work is associated with the MSE-in-Worker spec issue:
w3c#175