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

Review URL: https://codereview.chromium.org/866773002
2 files changed