Fix build warnings in SkAlphaThresholdFilter

BUG=None
R=bsalomon@google.com

Author: zork@chromium.org

Review URL: https://codereview.chromium.org/101763010

git-svn-id: http://skia.googlecode.com/svn/trunk@12937 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed