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In preparation for moving the MediaSource core interface into the
MediaSourceAttachment interface, this change factors out a necessary
class for retaining the existing same-thread Oilpan collection of idle
unreferenced attached HTMLMediaElement+MediaSource object groups.

Specifically, adds MediaSourceTracer to specialize further in later
changes in modules concrete implementations how to handle Oilpan tracing
of an HTMLMediaElement and a MediaSource in same- versus cross-thread
attachments. Includes same-thread concrete specialization in this
change. Having this as a separate entity reduces complexity and
fragility on both of those sides of the attachment. Note that a same
thread non-oilpan concrete attachment cannot use "Member" since the
attachment class is unmanaged. Hence the need for this MediaSourceTracer
to encapsulate that "Member" linkage.

BUG=878133

Change-Id: I4153ade37a1d4303af2604c2c2e18f701a79834d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363703Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800166}