Plumb the use of GrBackendRenderTarget throughout Skia

Bug: skia:
Change-Id: Ib99a58d9552f5c7b8d77c09dcc72fa88326c26aa
Reviewed-on: https://skia-review.googlesource.com/14148
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
22 files changed