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Instead of manipulating the media element's audio and video tracks
directly from the SourceBuffer, this change moves that into the
MediaSourceAttachmentSupplement interface.

Currently, TrackBase and TrackListBase usage by HTMLMediaElement and
SourceBuffer require ownership and operation by the main thread, and
further require knowledge of the main-thread-owned media element for
use in scheduling events on the main thread. For cross-thread MSE-in-
Workers attachments, to prevent regressing BackgroundVideoOptimization,
the media element will need knowledge of the worker-MSE audio and video
track identifications used by the underlying media pipeline.

This change supports future signalling of the media element to add or
remove such audio and video tracks via new methods in
MediaSourceAttachmentSupplement. Existing same-thread behavior is
maintained by new implementations of these methods in the
SameThreadMediaSourceAttachment. SourceBuffer is updated to understand
which of the methods to use, as until track ownership and operation
in worker context is supported, workarounds in SourceBuffer and the
attachment will be needed.

Later changes will include the cross-thread attachment implementation
of the media-element track addition and removal to actually perform
the signalling.

This change retains the pre-existing experimental AudioVideoTracks
ability of MSE in Chrome for same-thread-attachments, but prevents
(with a CHECK failure, since IDL prohibits alternatives) retrieval by
the app of a SourceBuffer's audioTracks or videoTracks when the MSE is
in a worker context.

BUG=878133

Change-Id: I209ce61915de2e4182807539013606322ece8f67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427566Reviewed-by: Will Cassella <cassew@google.com>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812984}