Make SkImage_Gpu MakePromise interfaces consistent

Bug: skia:7903
Change-Id: Ia14cdafcfc26f75fde4296e7e93ce6f56a98400f
Reviewed-on: https://skia-review.googlesource.com/c/161827
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed