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Replaces remaining direct usage of the attached HTMLMediaElement from
MediaSource and SourceBuffer, relying instead on the attachment to
moderate that communication.

Specifically, this change:

* Adds attachment helpers to enable SourceBuffers to create their audio
  and video tracklist members, including a same-thread attachment
  implementation. Cross-thread attachment implementation of this is
  deferred until TrackListBase and TrackBase can be owned off the main
  thread (in a worker thread), and until they can function without
  themselves directly accessing an HTMLMediaElement.

* Removes superfluous usage in SourceBuffer of `source_->MediaElement()`
  in DCHECKs; other DCHECKs already suffice.

* Removes MediaSource::MediaElement() and `attached_element_`. Removes
  their superfluous implementation and usage from MediaSource.

* Fixes outdated assumptions in various DCHECKs that all attachments use
  a tracer. Only same-thread attachments use a tracer.

BUG=878133

Change-Id: I346b320804a0ffbf731788b7ea7c344588f660b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439677
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Will Cassella <cassew@google.com>
Cr-Commit-Position: refs/heads/master@{#814329}