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This adds a flag to disable:
- window.OffscreenCanvas
- Context2DCreation.desynchronize.
- HTMLCanvasElement.transferControlToOffscreen

This is a reland to add WebView layout test expectations.

TBR=alexmos,chrishtr,wfh,boliu

Bug: 940953
Change-Id: I1973c94ced693f0e91785c5197c0d9173e467af8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1743098Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685334}