change setAlphaType to not modify the pixelref's genID

BUG=skia:
R=bsalomon@google.com, halcanary@google.com

Review URL: https://codereview.chromium.org/137263009

git-svn-id: http://skia.googlecode.com/svn/trunk@13219 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed