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According to the comment, this extra copy was needed when the bitmaps in a platform-canvas
wasn't ref-countable. That is no longer the case (hasn't been true for quite a while AFAIK)
so this CL removes that code.

BUG=

Review-Url: https://codereview.chromium.org/2799283003
Cr-Commit-Position: refs/heads/master@{#462845}