Use an image generator to back SkPictureShader tiles.

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