Make CanvasResource RefCounted

This is a step towards unifying OffscreenCanvasResourceProvider
with CanvasResourceProvider, which will eventually allow low
latency canvas rendering to use a single buffered GpuMemoryBuffer

BUG=788439, 776801

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ie9f04e7920e7d74c229c4d45c34c371ab5d06e36
Reviewed-on: https://chromium-review.googlesource.com/800670
Commit-Queue: Justin Novosad <junov@chromium.org>
Reviewed-by: Olivia Lai <xlai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520586}
4 files changed