add writePixels to SkBitmap

BUG=skia:

Change-Id: I6f4db9ddc0364d9785e0f1794d86b73e66845ea9
Reviewed-on: https://skia-review.googlesource.com/6593
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
3 files changed