fix casting issue with or operator

R=reed@google.com, epoger@google.com
TBR=reed@google.com
NOTREECHECKS=true
NOTRY=true

Author: djsollen@google.com

Review URL: https://codereview.chromium.org/305253002

git-svn-id: http://skia.googlecode.com/svn/trunk/include@15011 2bbb7eff-a529-9590-31e7-b0007b416f81