Make mock GrContext unit testable.

Bug: skia:
Change-Id: I959122f1f2c390832ab1033bcdbdd2ca6cfc0419
Reviewed-on: https://skia-review.googlesource.com/20699
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
16 files changed