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

cc/resources/eviction_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_EVICTION_TILE_PRIORITY_QUEUE_H_
 #define CC_RESOURCES_EVICTION_TILE_PRIORITY_QUEUE_H_
 
 #include <set>
 #include <utility>
 #include <vector>
 
 #include "cc/base/cc_export.h"
 #include "cc/layers/picture_layer_impl.h"
 #include "cc/resources/tile_priority.h"
 #include "cc/resources/tiling_set_eviction_queue.h"
 
 namespace cc {
+class PrioritizedTile;
 
 class CC_EXPORT EvictionTilePriorityQueue {
  public:
   EvictionTilePriorityQueue();
   ~EvictionTilePriorityQueue();
 
   void Build(const std::vector<PictureLayerImpl*>& active_layers,
              const std::vector<PictureLayerImpl*>& pending_layers,
              TreePriority tree_priority);
 
   bool IsEmpty() const;
-  Tile* Top();
+  const PrioritizedTile& Top() const;
   void Pop();
 
  private:
   ScopedPtrVector<TilingSetEvictionQueue>& GetNextQueues();
+  const ScopedPtrVector<TilingSetEvictionQueue>& GetNextQueues() const;
 
   ScopedPtrVector<TilingSetEvictionQueue> active_queues_;
   ScopedPtrVector<TilingSetEvictionQueue> pending_queues_;
   TreePriority tree_priority_;
 
   DISALLOW_COPY_AND_ASSIGN(EvictionTilePriorityQueue);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_EVICTION_TILE_PRIORITY_QUEUE_H_