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After processing overlay candidates we used to remove the overlay rects
from the damage rect.
The assumption was that all the overlays would be opaque.

Since we started allowing alpha blended overaly, we should avoid
subtracting from the damage when blending might be allowed.
Otherwise any element animating below a transparent overlay might
not be updated.

We haven't noticed any problem so far since partial update was disabled
on Mali. crrev.com/2829543003 enables empty swap, surfacing the problem
when the damage region is a subset of a transparent overlay.

BUG=705290
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2842383003
Cr-Commit-Position: refs/heads/master@{#467727}