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Previously the mask was applied only to the first PIP window. This CL
fixes this bug.

Bug: 902637
Test: Tested manually with the steps on the issue
Test: Added test passes

Change-Id: I4f86fa1713a861243ab3c87d051ff491f7b6bd32
Reviewed-on: https://chromium-review.googlesource.com/c/1322188Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609924}