Hide GrGpuResourceRef from prying eyes

Change-Id: I5535b8cc8e82ed4b6fe847b96b417531b4bae348
Reviewed-on: https://skia-review.googlesource.com/19967
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed