Attempt to fix -1->unsigned warning on mac 10.6

TBR=bungeman@google.com

Review URL: https://codereview.chromium.org/199433002

git-svn-id: http://skia.googlecode.com/svn/trunk@13796 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed