Make OffscreenCanvas use the ResourceProvider from CanvasResourceHost.

With this CL, both <canvas> and OffscreenCanvas use the same
CanvasResourceProvider reference.

BUG=788439

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I52b8a91e18bfe2d0c003a38cb8a966b1abea7c50
Reviewed-on: https://chromium-review.googlesource.com/1079463
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563392}
7 files changed