Fold SkImageCacherator into SkImage_Lazy

Fold SkImageCacherator into SkImage_Lazy

SkImageCacherator still exists, but only as an interface implemented
(solely) by SkImage_Lazy. The only external clients are
GrImageTextureMaker and SkImage_Gpu::getDeferredTextureImageData.

This is probably an improvement, but doesn't go as far as I'd hoped.

Bug: skia:
Change-Id: I6812badfabb6924b025621b21af00cbde9c16cac
Reviewed-on: https://skia-review.googlesource.com/14371
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>