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This CL fixes some thread-safety issues in our video decoder tests. These might
be responsible for some rare crashes we've been seeing when running the tests on
our test infrastructure, but are not reproducible locally.

Most callbacks will be triggered on the same thread that scheduled these, but
this is not always the case. However WeakPtrs are not thread-safe, so this CL
introduces functors that guarantee that WeakPtrs are only dereferenced on the
thread that created them.

This CL also adds an additional sequence check to make sure events are sent on
the correct thread, and a comment is added about the use of base::Unretained for
the video player event callback.

TEST=./video_decode_accelerator_tests on samus

BUG=981718

Change-Id: If04268953f9be5dabc4ec11759fb3223f45055cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1843998
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704467}