Change ui::Layer API to take TransferableResource

Instead of TextureMailbox, which it converts, it can take a
TransferableResource directly. This leaves TextureMailbox as only
used for Gpu-backed CopyOutputResults. Code that uses that as an
input for ui::Layer does the conversion itself for now.

R=piman@chromium.org
TBR=reveman@chromium.org

Bug: 769423
Change-Id: Id4ab7462d7f7c98dd834f746605e520be23696e7
Reviewed-on: https://chromium-review.googlesource.com/780241


Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518288}