commit to xfermode objects being immutable

BUG=
R=djsollen@google.com, mtklein@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/25968004

git-svn-id: http://skia.googlecode.com/svn/trunk/src@11614 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed