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* It observes widget's destruction instead of observing
 compositor's in CompositorChecker. This allow CAR to continue running
 even if the widget's compositor changes.
* Re-enable on chromeos

Bug: 1019536
Test: Covered by unittests.
Change-Id: Ia3a3201408b93d70b98339e79b7e9ffd6db22e56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890493Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712641}