Do some refactoring of GrAHardwareBufferImageGenerator.

This is just some pre-work to split out from the change to make the
GrAHardwareBufferImageGenerator use lazy proxies.

Bug: skia:
Change-Id: I655d0c20070cff391d2bb9487b7c73253493cf71
Reviewed-on: https://skia-review.googlesource.com/148060
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed