Fix 'i-i' to 'i-1' in SkCanvasStack.

Pointed out in http://www.viva64.com/en/b/0225/ .

R=djsollen@google.com

Review URL: https://codereview.chromium.org/99753002

git-svn-id: http://skia.googlecode.com/svn/trunk@12439 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed