implemented GrMtlGpuCommandBuffer for draws in Metal gpu backend

Bug: skia:
Change-Id: Ifc20a7734a02fe48ef79af201b4ebc882aedefb3
Reviewed-on: https://skia-review.googlesource.com/146382
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
6 files changed