Some GrSemaphore cleanup

Remove unused GrResourceProvider functions that transfer context
ownership.

Return GrBackendSemaphore by value, make method public.

Change-Id: I1fe83b661cab82a54dde33b4b649579ea01be97c
Reviewed-on: https://skia-review.googlesource.com/150363
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
7 files changed