This CL moves video encode accelerator IPC messages to GPU IO thread
instead of the main thread. This helps stabilize frame rate as well
as reduce jitter on Windows. Currently, a lot of these calls get huge
delays and results in dropped frames.

In order to do this with respect to each platform,
TryToSetupEncodeOnSeparateThread() is added to VideoEncodeAccelerator
interface. If this method return false, we keep all IPC messages on
main thread like before. It returns false by default.

Note: Initially, I only moved AcceleratedVideoEncoderMsg_Encode call
to IO thread, but then we started waiting for output buffers for a
long time and again stayed below 30 fps. Therefore I moved all three
functions to IO thread and reached ~30 fps, even when switching between
tabs.

BUG=657217, 649275
TEST=RunsAudioVideoCall60SecsAndLogsInternalMetricsH264 browser test
on Windows result in stable 30 fps.

Review-Url: https://codereview.chromium.org/2427053002
Cr-Commit-Position: refs/heads/master@{#430882}