Add guard around GrVkBackendContext::Create function.

Just want to make sure the use of this doesn't leak out to other users
as I work on removing it.

Bug: skia:
Change-Id: I5ecf11615db64bd489065e6f67e0e1d4bb3298eb
Reviewed-on: https://skia-review.googlesource.com/132831
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed