use ClipOp instead of SkRegion for clipping

src/utils/SkGatherPixelRefsAndRects.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkGatherPixelRefsAndRects.h"
 #include "SkNoSaveLayerCanvas.h"
 #include "SkPictureUtils.h"
 
 void SkPictureUtils::GatherPixelRefsAndRects(SkPicture* pict,
                                              SkPictureUtils::SkPixelRefContainer* prCont) {
     if (pict->cullRect().isEmpty()) {
         return ;
     }
 
     SkGatherPixelRefsAndRectsDevice device(SkScalarCeilToInt(pict->cullRect().width()), 
                                            SkScalarCeilToInt(pict->cullRect().height()), 
                                            prCont);
     SkNoSaveLayerCanvas canvas(&device);
 
-    canvas.clipRect(pict->cullRect(), SkRegion::kIntersect_Op, false);
+    canvas.clipRect(pict->cullRect());
     canvas.drawPicture(pict);
 }