Alter createTestingOnlyBackendTexture methods

This intended to bring this API more into line with the proposed GrBackendObject API with an eye towards replacing the former with the latter.

TBR=bsalomon@google.com
Change-Id: I4367f03fb10fff788749f21c4843060111a6df1c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213220
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
46 files changed