Move GrBackendRenderTarget over to new system of getting backed infos

Bug: skia:
Change-Id: I3927390894715e8424b3d0240dad3ee6cd03dc38
Reviewed-on: https://skia-review.googlesource.com/120181
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
8 files changed