implemented onWritePixels and related tests for metal gpu backend

Bug: skia:
Change-Id: Iab8bc8611a9de8afcf0eaeb58a8ef46fdc38d4f1
Reviewed-on: https://skia-review.googlesource.com/140571
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed