Make writeRenderTargetPixels use draw buffer rather than GrGpu

Make writeRenderTargetPixels use draw buffer rather than GrGpu

Committed: https://skia.googlesource.com/skia/+/656f2cd5588fabde2bbc2bd42763beb1347fc491

[bsalomon, 2014-09-23 15:18:48]

bsalomon@google.com changed reviewers:
+ robertshield@google.com

[bsalomon, 2014-09-23 15:20:02]

bsalomon@google.com changed reviewers:
+ robertphillips@google.com
- robertshield@google.com

[bsalomon, 2014-09-26 16:57:11]

Updated to not flush and rebased.

[robertphillips, 2014-09-29 12:03:33]

lgtm

[bsalomon, 2014-09-29 13:54:10]

The CQ bit was checked by bsalomon@google.com

[commit-bot: I haz the power, 2014-09-29 13:55:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/586393005/80001

[commit-bot: I haz the power, 2014-09-29 13:59:18]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-29 13:59:19]

Try jobs failed on following builders:
  Build-Mac10.8-Clang-x86_64-Release-Trybot on tryserver.skia
(http://108.170.220.76:10117/builders/Build-Mac10.8-Clang-x86_64-Release-Trybo...)
  Build-Ubuntu13.10-GCC4.8-Arm7-Debug-Android-Trybot on tryserver.skia
(http://108.170.220.76:10117/builders/Build-Ubuntu13.10-GCC4.8-Arm7-Debug-Andr...)
  Build-Ubuntu13.10-GCC4.8-x86_64-Release-Trybot on tryserver.skia
(http://108.170.220.76:10117/builders/Build-Ubuntu13.10-GCC4.8-x86_64-Release-...)
  Test-Ubuntu13.10-GCE-NoGPU-x86_64-Debug-Trybot on tryserver.skia
(http://108.170.220.120:10117/builders/Test-Ubuntu13.10-GCE-NoGPU-x86_64-Debug...)

[bsalomon, 2014-09-29 17:53:33]

The CQ bit was checked by bsalomon@google.com

[commit-bot: I haz the power, 2014-09-29 17:54:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/586393005/100001

[commit-bot: I haz the power, 2014-09-29 18:02:02]

Committed patchset #6 (id:100001) as 656f2cd5588fabde2bbc2bd42763beb1347fc491