Re-enable canvas optimization for getImageData with new codepath

This change re-enables the optimization that makes canvases switch
out of gpu-acceleration when getImageData is called.  The old
implementation was scrapped in favor of the new
ImageBuffer::disableAcceleration method that was introduced by
https://crrev.com/98739b054a5b28cc607d688b2e9e582456bbb20e

BUG=606688

Review-Url: https://codereview.chromium.org/2171513003
Cr-Commit-Position: refs/heads/master@{#406950}
6 files changed