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Many of our current issues turned out to be due to missing shutdown
logic which caused the GPU thread to shutdown before the compositor,
leading to a number of DCHECKs, and a hang if those were removed.

This change allows the CompositorImpl to tear down before the GPU proc.

Bug: 851643
Change-Id: Ieb96da9b7132e4ccf8ebf71dc0ee920bfbeb4217
Reviewed-on: https://chromium-review.googlesource.com/1115875Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570880}