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We are already disabling GPU acceleration to avoid readbacks caused
by calls to getImageData. This change applies the same principle
to canvas-to-canvas drawImage calls in order to avoid probable
future readbacks.

BUG=652126, 651517, 650116, 642539, 640144

Review-Url: https://codereview.chromium.org/2388293002
Cr-Commit-Position: refs/heads/master@{#422848}