cc: Separate the priority from the tile and put in new PrioritizedTile

cc/resources/raster_tile_priority_queue.h

 // Copyright 2014 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_RESOURCES_RASTER_TILE_PRIORITY_QUEUE_H_
 #define CC_RESOURCES_RASTER_TILE_PRIORITY_QUEUE_H_
 
 #include <vector>
 
 #include "cc/base/cc_export.h"
 #include "cc/layers/picture_layer_impl.h"
 #include "cc/resources/tile_priority.h"
 
 namespace cc {
-class Tile;
+class PrioritizedTile;
 
 class CC_EXPORT RasterTilePriorityQueue {
  public:
   enum class Type { ALL, REQUIRED_FOR_ACTIVATION, REQUIRED_FOR_DRAW };
 
   static scoped_ptr<RasterTilePriorityQueue> Create(
       const std::vector<PictureLayerImpl*>& active_layers,
       const std::vector<PictureLayerImpl*>& pending_layers,
       TreePriority tree_priority,
       Type type);
 
   virtual ~RasterTilePriorityQueue() {}
 
   virtual bool IsEmpty() const = 0;
-  virtual Tile* Top() = 0;
+  virtual const PrioritizedTile& Top() const = 0;
   virtual void Pop() = 0;
 
  protected:
   RasterTilePriorityQueue() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(RasterTilePriorityQueue);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_RASTER_TILE_PRIORITY_QUEUE_H_