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We will only do the optimization for the root render pass. If it's not root,
the overlay damage rect should be added first. This way it has a better chance
to be carved out, just the way it was before the optimization CL.

Bug:868561

Change-Id: I930307cb12ae3bd4fffd79a6311635e501ee1699
Reviewed-on: https://chromium-review.googlesource.com/c/1488037
Commit-Queue: Maggie Chen <magchen@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636254}