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This CL simplifies and cleans up the way ConfigureDisplayTask manages
its state over sync/async callbacks to reduce complexity and race-risk
by tightening up the way calls to Run() and OnConfigure() are handled.

In addition, this CL adds a test to ensure that we attempt to
reconfigure displays when any of them fail, not just the last one.

Finally, it removes ConfigureDisplaysTaskTest.ConfigureWithNoDisplays,
since this case should never occur in reality.

TEST=display_unittests

Change-Id: Icf34b00205e980df2bb5facbe6da5387a3e9dce3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422559
Commit-Queue: Gil Dekel <gildekel@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814355}