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

cc/resources/raster_tile_priority_queue_required.h

 // Copyright 2015 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_REQUIRED_H_
 #define CC_RESOURCES_RASTER_TILE_PRIORITY_QUEUE_REQUIRED_H_
 
 #include <vector>
 
 #include "cc/layers/picture_layer_impl.h"
 #include "cc/resources/raster_tile_priority_queue.h"
 #include "cc/resources/tiling_set_raster_queue_required.h"
 
 namespace cc {
-class Tile;
+class PrioritizedTile;
 
 class RasterTilePriorityQueueRequired : public RasterTilePriorityQueue {
  public:
   RasterTilePriorityQueueRequired();
   ~RasterTilePriorityQueueRequired() override;
 
   bool IsEmpty() const override;
-  Tile* Top() override;
+  const PrioritizedTile& Top() const override;
   void Pop() override;
 
  private:
   friend class RasterTilePriorityQueue;
 
   void Build(const std::vector<PictureLayerImpl*>& active_layers,
              const std::vector<PictureLayerImpl*>& pending_layers,
              Type type);
   void BuildRequiredForDraw(
       const std::vector<PictureLayerImpl*>& active_layers);
   void BuildRequiredForActivation(
       const std::vector<PictureLayerImpl*>& active_layers,
       const std::vector<PictureLayerImpl*>& pending_layers);
 
   ScopedPtrVector<TilingSetRasterQueueRequired> tiling_set_queues_;
 
   DISALLOW_COPY_AND_ASSIGN(RasterTilePriorityQueueRequired);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_RASTER_TILE_PRIORITY_QUEUE_REQUIRED_H_