Optimize painting WebGL to the canvas again.

Removing the earlier optimization in PaintRenderingResultsToCanvas
induced too large a performance regression for certain cases, such as
drawing static WebGL content to another canvas.

Bug: 897140
Change-Id: Ibf734ba53f5171f893913a487f74778761123b18
Reviewed-on: https://chromium-review.googlesource.com/c/1295334
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602017}
2 files changed