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Single buffer mode for pepper relied on TakeFromBuffer associating
different mailboxes to the same texture.
This does not work anymore after https://crrev.com/c/1097696 and
single buffer nacl plugins now sends invalid mailboxes to the compositor.

This CL fixes the issue making sure that Graphics3D is aware of the
single buffer mode and it will associate only one mailbox, only once,
to the front buffer.

Bug: 866643, b/111601347
Test: http://nacl-latency.firebaseapp.com runs without flickering

Change-Id: I8625fbb87bf3e9503068681bed681c88a068a123
Reviewed-on: https://chromium-review.googlesource.com/1144486
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579123}