add optional SkAlphaType parameter to notifyPixelsChanged

R=scroggo@google.com, halcanary@google.com, robertphillips@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/147213002

git-svn-id: http://skia.googlecode.com/svn/trunk@13192 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed