added Metal support to GrBackendSurface

Bug: skia:
Change-Id: I58c7307f41df4694b0b46014bedd681adefed265
Reviewed-on: https://skia-review.googlesource.com/137891
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
7 files changed