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Currently, VizMainImpl::ExitProcess has a workaround to avoid deadlock
when shutting down the gpu process. Unfortunately, this is only wired up
to one of the ways to shut down. Other shutdown calls originating from
ChildProcessImpl miss this logic and hit the same deadlock.

This change passes a helper to ChildProcessImpl, allowing it to correctly
handle shutdown. This helper posts a task to the GPU main thread before
starting shutdown to ensure both that we're on the right thread as well
as that we're post-init.

Bug: 901396
Change-Id: I58cff914def5c05f7b9e6b6f58cb319153d685dd
Reviewed-on: https://chromium-review.googlesource.com/c/1315538Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605043}