implement the logic to set tile priorities based on current matrix

cc/picture_layer_impl.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_PICTURE_LAYER_IMPL_H_
 #define CC_PICTURE_LAYER_IMPL_H_
 
 #include "cc/layer_impl.h"
 #include "cc/picture_layer_tiling.h"
 #include "cc/picture_layer_tiling_set.h"
@@ skipping 26 matching lines @@
 
   // PushPropertiesTo active tree => pending tree
   void SyncFromActiveLayer(const PictureLayerImpl* other);
 
 protected:
   PictureLayerImpl(int id);
 
   PictureLayerTilingSet tilings_;
   scoped_refptr<PicturePileImpl> pile_;
 
+  gfx::Transform last_screen_space_transform_;
+  double last_update_time_;
+  gfx::Size last_bounds_;
+  gfx::Size last_content_bounds_;
+  float last_content_scale_x_;
+  float last_content_scale_y_;
+
   friend class PictureLayer;
   DISALLOW_COPY_AND_ASSIGN(PictureLayerImpl);
 };
 
 }
 
 #endif  // CC_PICTURE_LAYER_IMPL_H_