cc: Optimise shared raster tile handling in raster tile priority queue.

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 <utility>
 #include <vector>
 
@@ skipping 12 matching lines @@
     ~PairedPictureLayerQueue();
 
     bool IsEmpty() const;
     Tile* Top(TreePriority tree_priority);
     void Pop(TreePriority tree_priority);
 
     WhichTree NextTileIteratorTree(TreePriority tree_priority) const;
 
     PictureLayerImpl::LayerRasterTileIterator active_iterator;
     PictureLayerImpl::LayerRasterTileIterator pending_iterator;
+    bool has_both_layers;
 
+#if DCHECK_IS_ON

[reveman, 2014/09/05 17:08:32]

It would be better to always compile this so that it would work with
--enable-dchecks. Does having an always empty container here impact performance?

[USE eero AT chromium.org, 2014/09/08 14:08:17]

Done.
Not measurable impact.

     // TODO(vmpstr): Investigate removing this.

[reveman, 2014/09/05 17:08:32]

This TODO is not valid anymore, right? How about we update this to a comment
that explains what it is now used for?

[USE eero AT chromium.org, 2014/09/08 14:08:17]

Done.

     std::vector<Tile*> returned_shared_tiles;

[reveman, 2014/09/05 17:08:32]

Now that this is only used for debug checks, can we make it a std::set? Makes
the intent more clear and the code a bit cleaner, I think.

[USE eero AT chromium.org, 2014/09/08 14:08:17]

Yes, consumes more memory than std::vector but compared to the memory used by
the two layer raster tile iterators that is nothing.
If you say so.

+#endif
   };
 
   RasterTilePriorityQueue();
   ~RasterTilePriorityQueue();
 
   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(RasterTilePriorityQueue);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_RASTER_TILE_PRIORITY_QUEUE_H_