Use fast path of mailbox generation in RGVAF::CreateTextures.

Use fast path of mailbox generation in RGVAF::CreateTextures.

This reduces the latency of CreateTextures (8 textures) from around
1000ms to 1ms for HW decode WebRTC on Chromebook Daisy.

GLES2Implementation doesn't currently have the right hook to make
mailbox generation fast, though WebGraphicsContext3DCommandBufferImpl
does. The fast path for mailbox generation only exists in content,
that GLES2Implementation, in gpu, can't directly access. So
GLES2Implementation has to go through command buffers, which is bad
for round-trips.

BUG=170345
TEST=Run http://apprtc.appspot.com/?debug=loopback on Chromebook Daisy.
     Play local hardware-accelerated videos on Daisy.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=221285

[Ami GONE FROM CHROMIUM, 2013-09-03 18:55:05]

I'm not competent to review this change's meat since I don't understand any of
the newfangled mailbox business.  Rubberstamp LGTM in case you need it for
OWNERS purposes, subject to piman's review.

It might be useful to put the justification (from the CL description) in a NOTE
comment above the new code.

[piman, 2013-09-03 20:19:28]

LGTM, maybe add a comment so that someone doesn't try to "clean up" for
consistency.

[wuchengli, 2013-09-04 07:28:58]

Added comment. Submitting now.

[commit-bot: I haz the power, 2013-09-04 16:36:05]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/wuchengli@chromium.org/23567012/9001

[commit-bot: I haz the power, 2013-09-04 22:50:30]

Change committed as 221285