implement onreadpixels for metal gpu backend

Bug: skia:
Change-Id: I1645f7e4e65766558f2b6744ce28d5f98ce852c6
Reviewed-on: https://skia-review.googlesource.com/138982
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
3 files changed