Make OffscreenCanvas 2d contexts use CanvasResource

This change is a step toward unifying the canvas and OffscreenCanvas
presentation code paths by making offscreen 2d contexts use
CanvasResource for pushing frames.

BUG=788439
TBR=zmo@chromium.org
NOTRY=true

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
Change-Id: Icc138c8ab53a28c16d9502ef774064acb0087ac5
Reviewed-on: https://chromium-review.googlesource.com/1135580
Commit-Queue: Justin Novosad <junov@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576612}
8 files changed