Also add a GetProc function to the GrVkBackendContext which will be used
to create the GrVkInterface.
This change (and updating clients to use it), will allow us to move GrVkInterface
out of public which is needed to fix vulkan header issues.
Bug: skia:
Change-Id: Id8067943ae27cec8cad29fd31b05f0b8387412d4
Reviewed-on: https://skia-review.googlesource.com/140783
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
c8cd45aaf4