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* MediaMemoryDumper - simple wrapper over MemoryDumpProvider
that takes care of registering/unregestering and calling a external
callback.

* Two instances of MediaMemoryDumper inside of WebMediaPlayerImpl.
One runs on the media thread and counts Demuxer's memory,
the other runs on the main thread and handles everything else
(video, audio, data source).

* The fact that demuxers need to be createad and destroyed on the
main thread, but used only on the media thread makes things interesting.

Bug: 1056839
Change-Id: I9a3386626789ddd1079d4d6759243d7622dea39c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084083
Commit-Queue: Eugene Zemtsov <eugene@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747585}