add writePixels to SkSurface --> so we can remove it from canvas

Bug: skia:
Change-Id: Ib35d7bdc113c3fd9f6dfacb42ec61aaa448cb165
Reviewed-on: https://skia-review.googlesource.com/105623
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
11 files changed