Move GrcommandBufferRef.h to more generic GrRefCnt.h

I plan to add at least one more gr_* class, so I figure it makes sense
to just keep all this similar things in one place.

Bug: skia:11136
Change-Id: I96d24dd094731e9694201e012fec37926cce564d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/350639
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
3 files changed