Clean up onTransferPixels

Bug: skia:5126
Change-Id: I323c50e7854744302007b4ae7bd25e5742c14cbc
Reviewed-on: https://skia-review.googlesource.com/19055
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
11 files changed