[WIP] cc: Use reverse spiral iterator in tiling eviction.

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"
 
 namespace cc {
 
 class CC_EXPORT EvictionTilePriorityQueue {
  public:
   struct PairedPictureLayerQueue {
     PairedPictureLayerQueue();
     PairedPictureLayerQueue(const PictureLayerImpl::Pair& layer_pair,
                             TreePriority tree_priority);
     ~PairedPictureLayerQueue();
 
     bool IsEmpty() const;
     Tile* Top(TreePriority tree_priority);
     void Pop(TreePriority tree_priority);
 
     WhichTree NextTileIteratorTree(TreePriority tree_priority) const;
 
     PictureLayerImpl::LayerEvictionTileIterator active_iterator;
     PictureLayerImpl::LayerEvictionTileIterator pending_iterator;
 
-    // TODO(vmpstr): Investigate removing this.
-    std::vector<Tile*> returned_shared_tiles;
+    // Set of returned tiles (excluding the current one) for DCHECKing.
+    std::set<const Tile*> returned_tiles_for_debug;
   };
 
   EvictionTilePriorityQueue();
   ~EvictionTilePriorityQueue();
 
   void Build(const std::vector<PictureLayerImpl::Pair>& paired_layers,
              TreePriority tree_priority);
   void Reset();
 
   bool IsEmpty() const;
   Tile* Top();
   void Pop();
 
  private:
   // TODO(vmpstr): This is potentially unnecessary if it becomes the case that
   // PairedPictureLayerQueue is fast enough to copy. In that case, we can use
   // objects directly (ie std::vector<PairedPictureLayerQueue>).
   ScopedPtrVector<PairedPictureLayerQueue> paired_queues_;
   TreePriority tree_priority_;
 
   DISALLOW_COPY_AND_ASSIGN(EvictionTilePriorityQueue);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_EVICTION_TILE_PRIORITY_QUEUE_H_