Revert of Canvas.toDataURL to use SkBitmap::readPixels to avoid uninitialized memory

Revert of Canvas.toDataURL to use SkBitmap::readPixels to avoid uninitialized memory (patchset #4 id:60001 of https://codereview.chromium.org/1234083003/)

Reason for revert:
ASan bots are failing on toDataURL() tests,

 http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20ASAN/builds/19488
 http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Oilpan%20ASAN/builds/2978

Original issue's description:
> Canvas.toDataURL to use SkBitmap::readPixels to avoid uninitialized memory
> 
> This change refactors toDataURL to make it read the canvas data into
> a pre-initialized buffer to avoid accessing unitialized memory in cases
> where a GPU readback fails silently.
> 
> BUG=504690
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=199131

TBR=senorblanco@chromium.org,junov@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=504690
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=199154

[sof, 2015-07-18 06:45:58]

Created Revert of Canvas.toDataURL to use SkBitmap::readPixels to avoid
uninitialized memory

[commit-bot: I haz the power, 2015-07-18 06:46:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1236173005/1

[commit-bot: I haz the power, 2015-07-18 06:46:29]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=199154