Use SkImage to manage mip map cache purging, not Bitmap/PixelRef

Lazy and GPU images manage the notification themselves, but raster
images now forward the notification request to the pixelRef.

Bug: skia:
Change-Id: I93e0ebd3749e3f7dbd506466299fde8923982435
Reviewed-on: https://skia-review.googlesource.com/c/161830
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
6 files changed