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Before, the background size was reconstituted from the layer tree by
taking the union of of black and video layers. Now, the actual size is
plumbed through.

Change-Id: I0cb643b350ef7e4fca06e3bfd419e24861a2e3c6
Reviewed-on: https://chromium-review.googlesource.com/1142962Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576521}