implement the logic to set tile priorities based on current matrix

cc/picture_layer_tiling.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_TILING_H_
 #define CC_PICTURE_LAYER_TILING_H_
 
 #include "base/basictypes.h"
 #include "base/hash_tables.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 68 matching lines @@
     int right_;
     int bottom_;
 
     friend class PictureLayerTiling;
   };
 
   Region OpaqueRegionInContentRect(const gfx::Rect&) const;
 
   void Reset() { return tiles_.clear(); }
 
+  void UpdateTilePriorities(const gfx::Size& device_view_port,
+                            float layer_content_scale_x,
+                            float layer_content_scale_y,
+                            const gfx::Transform& last_screen_transform,
+                            const gfx::Transform& current_screen_transform,
+                            double time_delta);
+
  protected:
   typedef std::pair<int, int> TileMapKey;
   typedef base::hash_map<TileMapKey, scoped_refptr<Tile> > TileMap;
 
   PictureLayerTiling(float contents_scale, gfx::Size tileSize);
   Tile* TileAt(int, int) const;
   void CreateTile(int i, int j);
 
   PictureLayerTilingClient* client_;
   float contents_scale_;
   gfx::Size layer_bounds_;
   TileMap tiles_;
   TilingData tiling_data_;
 
   friend class Iterator;
 };
 
 }  // namespace cc
 
 #endif  // CC_PICTURE_LAYER_TILING_H_