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In pre-CAP, when there is a full screen overlay layer, during paint, we
start from the overlay layer and ignore all other layers.

This CL let CAP do the equivalent thing: during paint, if there is
full screen overlay element, starts painting from the element instead of
from the top level frame.

Bug: 1031922
Change-Id: I7b0a25926e75bb12a475ecdb891f33d2634b8812
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2248487Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779500}