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To prepare for cross-thread HTMLMediaElement+MediaSource API
attachments, aka "MSE-in-Workers", this CL decouples the MediaSource
object from what is directly registered. A new type,
MediaSourceAttachment, is used to register available MediaSource
objects, and a simple concrete implementation is included in this type
to resolve the underlying MediaSource for use in actual attachment.

This CL is needed as a prerequisite for cross-thread attachments
because subsequent CLs are significantly complex. They will:
1) Move MediaSourceRegistry singleton creation to modules initializer to
ensure it is owned on the main thread, even if a worker thread was the
first to need it (see bullet 3, below).
2) Update MediaSourceAttachment to be an interface and move the
communication between HTMLMediaElement and the MediaSource to be done
via the interface with concrete implementation(s) in modules.
3) Add a cross-thread attachment implementation.  This part will be
complex, and will be better handled if the existing same-thread behavior
and the new cross-thread behavior are distinct implementations of a
common attachment interface. Further, having a non-oilpan type (the
concrete implementations of MediaSourceAttachment) as the registrable
enables the implementations to safely operate when lifetimes of either
side of the attachment vary. And such a non-oilpan type must be used as
the registrable if the registry singleton's internal initialization is
done prior to Oilpan being ready (during the modules initializer).
4) Refine the interface and concrete implementations for performance and
quality specific to the types of attachment being used.

BUG=878133

Change-Id: I8376f6483142d3653e5d39d76f5a245c80365b18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300804Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796549}