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Adds a supplemental interface in modules to extend the core
MediaSourceAttachment, so common methods in specialized attachments can
be defined without requiring them known by core. This reduces complexity
of the core side of the attachment interface.

Specifically, adds MediaSourceAttachmentSupplement, with a new method
(implemented in concrete SameThreadMediaSourceAttachment) to notify the
attached element that the duration of the resource has changed. Later
specialization in a cross-thread version of the attachment will need to
perform this notification asynchronously.

Lifts the Unregister() method and the persistent MediaSource OilPan
reference member from the concrete implementation into this common
modules attachment supplement's implementation, since these are expected
to remain the same across all concrete attachment implementations.

Updates SameThreadMediaSourceAttachment to give the MediaSource a
thread-safe refcounted scoped_refptr for the attachment during
StartAttachingToMediaElement(). MediaSource, like HTMLMediaElement does
already, is updated to know both the attachment and the
MediaSourceTracer while the attachment is active.

Starts using NotifyDurationChanged from the MediaSource object via the
attachment instead of directly talking to the attached media element
from the MediaSource.

Further extensions of this attachment interface will come in later
changes for similar scenarios, eventually enabling the dropping of the
|attached_element_| member from the MediaSource class, with all
interactions on the element eventually done via the attachment.

BUG=878133

Change-Id: I86206f740b62f9ba6dc6a1b9fd16a44aff21f49a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2387121Reviewed-by: Will Cassella <cassew@google.com>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804434}