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We should be able to avoid this with careful use of a trampoline
through the media thread after PipelineImpl::Stop() completes.

BUG=905506
TEST=passes cq.

Change-Id: I6a9c1a0e578c48a66af85588879346d26009d2ab
Reviewed-on: https://chromium-review.googlesource.com/c/1336659Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610327}