Encode frame number in pixel data for pose sync

This encodes the pose number of the drawn WebVR frame in a corner pixel
block, this is read by vr_shell.cc to use the corresponding pose when
submitting the frame to GVR for presentation.

Log data shows that the pose is 2-3 frames old by the time it is ready for
submission to GVR. This includes compositor buffering and SurfaceTexture
buffering. The patch helps in getting reprojection working as intended, but
can't do anything about the excessive latency.

BUG=389343
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2384593002
Cr-Commit-Position: refs/heads/master@{#423367}
13 files changed