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
diff --git a/src/utils/SkCanvasStack.cpp b/src/utils/SkCanvasStack.cpp
index db5a8b2..8951149 100644
--- a/src/utils/SkCanvasStack.cpp
+++ b/src/utils/SkCanvasStack.cpp
@@ -35,7 +35,7 @@
             localBounds.offset(origin - fCanvasData[i-1].origin);
 
             fCanvasData[i-1].requiredClip.op(localBounds, SkRegion::kDifference_Op);
-            fList[i-i]->clipRegion(fCanvasData[i-1].requiredClip);
+            fList[i-1]->clipRegion(fCanvasData[i-1].requiredClip);
         }
     }
     SkASSERT(fList.count() == fCanvasData.count());