Add cast to fix Win* compile complaint/failure

R=tomhudson@google.com, reed@google.com
TBR=tomhudson@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/100313005

git-svn-id: http://skia.googlecode.com/svn/trunk@12764 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed