Store the GrBackend we are using on GrContext

As we start adding more functionality to external clients and passing
data around, we will need to know what backend we are working with at
a layer higher that GrGpu.

TBR=bsalomon@google.com

Bug: skia:
Change-Id: Ifee527ea0c1046469401a39e0500f84a46bb3081
Reviewed-on: https://skia-review.googlesource.com/13275
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed