Add stubbed out GrContext

This CL mainly just:
   stores the GrContextOptions in GrContextThreadSafeProxy (so they can be passed on to a stubbed out GrContext)
   adds a method to create a stubbed out GrContext that has a GrStubGpu
      - the stubbed out GrContext isn't quite ready for prime time yet

Change-Id: I31be6763640e406c5963e6f0714489ac358339e4
Reviewed-on: https://skia-review.googlesource.com/79601
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
14 files changed