Revert of Use an image generator to back SkPictureShader tiles.

Revert of Use an image generator to back SkPictureShader tiles. (patchset #4 id:60001 of https://codereview.chromium.org/866773002/)

Reason for revert:
TSAN troubles.

Original issue's description:
> Use an image generator to back SkPictureShader tiles.
> 
> To avoid lifetime issues for tiles backed by discardable memory, use an
> image generator to re-generate them on the fly.
> 
> With this CL, we are now caching bitmap shaders wrapping discardable
> pixel ref bitmaps backed by picture image generators.
> 
> (the CL also includes some minor/unrelated SkPictureShader cleanup)
> 
> BUG=skia:3220
> R=reed@google.com,halcanary@google.com
> 
> Committed: https://skia.googlesource.com/skia/+/4739955e98ba86900a5bbac0e3661601427a54d4

TBR=reed@google.com,halcanary@google.com,mtklein@google.com,reed@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:3220
Committed: https://skia.googlesource.com/skia/+/f47e70712b964b5360a3fec9e5e84bb594fc1f02

[f(malita), 2015-02-26 01:45:52]

Created Revert of Use an image generator to back SkPictureShader tiles.

[commit-bot: I haz the power, 2015-02-26 01:46:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/956143002/1

[commit-bot: I haz the power, 2015-02-26 01:47:12]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/f47e70712b964b5360a3fec9e5e84bb594fc1f02