Specialize GrConvexPolyEffect for AA rects, use for AA clip rects.

src/gpu/effects/GrConvexPolyEffect.h

Index: src/gpu/effects/GrConvexPolyEffect.h
diff --git a/src/gpu/effects/GrConvexPolyEffect.h b/src/gpu/effects/GrConvexPolyEffect.h
index 035d9e95dda03c4a5e3f014399625f8788fcfd2b..434cebe96fa60fc1447fdcb7d7a0260ed044eb9f 100644
--- a/src/gpu/effects/GrConvexPolyEffect.h
+++ b/src/gpu/effects/GrConvexPolyEffect.h
@@ -61,6 +61,11 @@ public:
      */
     static GrEffectRef* Create(EdgeType, const SkPath&);
 
+    /**
+     * Creates an effect that clips against the rect.
+     */
+    static GrEffectRef* Create(EdgeType, const SkRect&);
+
     virtual ~GrConvexPolyEffect();
 
     static const char* Name() { return "ConvexPoly"; }