Make OffscreenCanvas use GpuMemoryBuffers for presentation

This CL enables GpuMemoryBuffer-backed pixel buffers for
OffscreenCanvases that are GPU-accelerated.  This should result
in the use of HW overlays on Mac and Chrome OS.

BUG=788439
TBR=zmo@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_layout_tests_slimming_paint_v2;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I6cb30f090b1175036a64f4f122162824023f42a0
Reviewed-on: https://chromium-review.googlesource.com/1091643
Commit-Queue: Justin Novosad <junov@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566871}
10 files changed