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Most of the changes replace canvas read pixels call in favor
of first getting the bitmap, and then doing a readpixels on that.

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2792133002
Cr-Commit-Position: refs/heads/master@{#462984}