Use CanvasResource more in OffscreenCanvasFrameDispatcher

Use CanvasResource instead of StaticBitmapImage in more places
in OffscreenCanvasFrameDispatcher. This change brings us closer to
unifying <canvas> and OffscreenCanvas resource management.

BUG=788439

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I2595acf5733015d1c892995181897be90d6619c5
Reviewed-on: https://chromium-review.googlesource.com/1076309
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562478}
3 files changed