Move mock gpu and caps, add Gr prefix, and create via GrContext::Create

Change-Id: I02427839f78372e337ed717b071c6baaa8ba4450
Reviewed-on: https://skia-review.googlesource.com/20622
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
9 files changed