Add clip effect for circles

src/gpu/effects/GrOvalEffect.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrOvalEffect_DEFINED
+#define GrOvalEffect_DEFINED
+
+#include "GrTypes.h"
+#include "GrTypesPriv.h"
+
+class GrEffectRef;
+struct SkRect;
+
+namespace GrOvalEffect {
+    /**
+     * Creates an effect that performs clipping against an oval.
+     */
+    GrEffectRef* Create(GrEffectEdgeType, const SkRect&);
+};
+
+#endif