Revert "Make GrVkBackendContext no longer derive from SkRefCnt."

This reverts commit 93ae2337732bf206e6ef4faecc6b30c3881e8359.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Make GrVkBackendContext no longer derive from SkRefCnt.
> 
> Also moves the helper Create functions to VkTestUtils since no clients
> are using them anymore.
> 
> Bug: skia:
> Change-Id: I7e8e4912e7ef6fb00a7e2a00407aed5e83211799
> Reviewed-on: https://skia-review.googlesource.com/135323
> Reviewed-by: Jim Van Verth <jvanverth@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Greg Daniel <egdaniel@google.com>

TBR=egdaniel@google.com,jvanverth@google.com,bsalomon@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: skia:
Change-Id: If7201917631dc22753ea3fa6e9d2984463e38e4c
Reviewed-on: https://skia-review.googlesource.com/137903
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
13 files changed