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Previously, we'd rely on an ApplicationStatusListener to deliver
background notifications. This was racy with sending the synchronous
command to tear down the RootCompositorFrameSink / Display, and if it
failed to trigger before teardown, we'd fail to disable the watchdog for
the expensive teardown operation.

This change ensures we always send the background signal before tearing
down the Display / RootCompositorFrameSink.

Bug: 899705
Change-Id: I2b5e5787f94ab1f392c5ec4c4063ef6664de2a3f
Reviewed-on: https://chromium-review.googlesource.com/c/1327468Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607052}