implemented uploading intial texture data for metal gpu backend

Bug: skia:
Change-Id: I76c820beb9b29b230a2f7d82fe8b6b141eee43d7
Reviewed-on: https://skia-review.googlesource.com/139163
Commit-Queue: Timothy Liang <timliang@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed