Add GrMockRenderTarget and mock backend render targets

Change-Id: I59673dd7d0015471b7a81aa0c237c67043892454
Reviewed-on: https://skia-review.googlesource.com/113427
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
10 files changed