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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.

This is an updated version of the last CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1336659

The difference is we now also preserve the RendererFactorySelector
until after renderer shutdown. This passes all the failing tests
I could find locally.

BUG=521176, 905506
TEST=passes cq.
R=sandersd

Change-Id: I86c4814a91ee6c41f20f6bb8a05859ac44171d7e
Reviewed-on: https://chromium-review.googlesource.com/c/1351791Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611305}