Use CanvasResource as a vehicle for OffscreenCanvas placeholder frames

This is an incremental step toward unifying <canvas> and
OffscreenCanvas around CanvasResource.

BUG=788439

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I2445e5cbe63f4ae329afd13cb07d3211e0f4de56
Reviewed-on: https://chromium-review.googlesource.com/1073634
Commit-Queue: Justin Novosad <junov@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562304}
10 files changed