Revert of Make 2D canvas disable gpu acceleration when getImageData is called (patchset #6 id:10000…

Revert of Make 2D canvas disable gpu acceleration when getImageData is called (patchset #6 id:100001 of https://codereview.chromium.org/2063473002/ )

Reason for revert:
Causing crash in mailbox release callback - bug 623101

Original issue's description:
> Make 2D canvas disable gpu acceleration when getImageData is called
>
> In order to optimize performance, GPU-accelerated canvases should
> permanently switch out of accalerated mode as soon as
> getImageData is called.  The cost of getImageData on the GPU is
> so prohibitively high that it almost always outways the benefit
> of acceleration.
>
> This is the first time we implement a SW/GPU switch that can happen
> after the first frame was presented, which may result in a small one
> time glitch due to rendering engine discrepancies. Let's see if we get
> any complaints about this.  If there are complaints, we may inhibit
> the switch based on what kind of content was rendered to the canvas.
>
> BUG=606688
>
> Committed: https://crrev.com/33732aec33c88cb18c212544da7d0a4ba53cd9ce
> Cr-Commit-Position: refs/heads/master@{#401751}

TBR=xlai@chromium.org,xidachen@chromium.org,junov@chromium.org
BUG=606688, 623101
Review-Url: https://codereview.chromium.org/2101823002
Cr-Commit-Position: refs/heads/master@{#402391}
(cherry picked from commit 27543ee6015a5d358e3abdb6ffb6121f6c847e31)

[xidachen, 2016-06-29 12:28:11]

The original CL has been reverted already. Could you please close this issue to
avoid any confusion in the future? Thank you.