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This is an optimization to reduce the size of damage rect in surface aggregation.
When clipping is on, we should clip the child surface damage rects before adding
them to the parent damage rect.

BUG=900422

Change-Id: Ia85f1d86bf4ba13ac66167c5de04c657d66d175c
Reviewed-on: https://chromium-review.googlesource.com/c/1309318Reviewed-by: enne <enne@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Maggie Chen <magchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605217}