Initial version working. \o/
Things missing:
- proper execution order of OFC GrContext and WebGL GrContext
- fix flush location
- security

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0079e522b9b0a4971ff73ca7b12b14f709f3c87b

commit 0079e522b9b0a4971ff73ca7b12b14f709f3c87b
Author: fserb <fserb@chromium.org>
Date: Thu May 11 06:24:59 2017

avoid calling host() directly for imageBuffer on 2D rendenring contexts

Follow up from previous CRCHost changes. Clean up.
Notice that discardImageBuffer is still called directly.

BUG=720062

Review-Url: https://codereview.chromium.org/2874463002
Cr-Commit-Position: refs/heads/master@{#470842}

[modify] https://crrev.com/0079e522b9b0a4971ff73ca7b12b14f709f3c87b/third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.cpp
[modify] https://crrev.com/0079e522b9b0a4971ff73ca7b12b14f709f3c87b/third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.cpp