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If the overlays are the only damages in the frame, we will get a zero
root damage rect with the new way of calculating damage rect
|use_overlay_damage_list_|, which is enabled by default.

The old code of |use_overlay_damage_list_| == false is still working. It
will be cleaned up once the new code is stable.

Bug:1117235

Change-Id: Ibbede44056973a5ba9e052292673bbf0bcb4a50a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434905
Commit-Queue: Maggie Chen <magchen@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: weiliangc <weiliangc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818723}