enabled gms and most unit tests for Metal gpu backend

Bug: skia:
Change-Id: Ib8a15931b217c6cb353e0af2d2752cb08a400f1d
Reviewed-on: https://skia-review.googlesource.com/146645
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Timothy Liang <timliang@google.com>
4 files changed