Refine srcObject's MediaSourceHandle behavior (#306)

* 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 (#305) and from lengthy
discussion on this change itself at
#306.

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

Multiple further changes made in response to review comments on this PR are
also included:

* MSE-in-Workers: Update attaching to a media element language
  Editorial only, based on PR comment:
  #306 (comment)

* MSE-in-Workers: Further update the attaching to a media element text
  Mostly editorial, in response to PR review comments.
  Non-editorial change:
  * Adds condition on [[Detached]] internal slot of a MediaSourceHandle,
    preventing successful attachment start if the handle instance is
    already transferred (detached).
  Editorial changes:
  * Replaces "been used beyond this point" text with a new MediaSource
    internal slot [[has ever been attached]]. Adds that slots definition
    and initialization in the MediaSource section, and also updates that
    internal slot to be true in the attaching steps.
  * Removes redundant-with-readyState-closed clause "being using beyond
    this point".

* MSE-in-Workers: Minor editorial update to extended resource fetch algorithm text
  Editorial change in response to PR comment:
  #306 (comment)
  Changes "these clarifications" to "these requirements"

* MSE-in-Workers: Another minor editorial update to extended resource fetch algorithm text
  Editorial change in response to PR comment:
  #306 (comment)
  Changes "do not apply" to "shall not apply"

* MSE-in-Workers: Minor editorial clarification to meaning of Detached
  In response to PR comment:
  #306 (comment)

* MSE-in-Workers: editorial, add reference to definition of "agent cluster"
  Editorial change in response to PR comment:
  #306 (comment)
  Adds normative reference to ECMAScript's definition of agent cluster,
  and adds an informative reference to related concepts in HTML where
  agent and agent cluster concepts from ECMAScript are integrated into
  HTML for Web APIs.

* MSE-in-Workers: Remove justification from srcObject setter extension note
  In response to PR comment:
  #306 (comment)

* MSE-in-Workers: Remove a couple redundant transfer steps
  Editorial, in response to PR comments:
  #306 (comment)
  #306 (comment)
  StructuredSerializeWithTransfer already moderates transfer success of
  Transferable objects like MediaSourceHandle with the object's
  [[Detached]] internal slot, so explicit extra steps to do this just for
  MediaSourceHandle are unnecessary and potentially confusing. Removed.

Author: wolenetz