Let client pass in full extension to GrVkBackendContext.

Bug: skia:
Change-Id: I772ab4ccbca0f4f7e7d429d6c421b07d97f0606f
Reviewed-on: https://skia-review.googlesource.com/131880
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
13 files changed