cc: Add const versions of operator* to layer iterators.

cc/layers/picture_layer_impl.h

 // Copyright 2012 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_LAYERS_PICTURE_LAYER_IMPL_H_
 #define CC_LAYERS_PICTURE_LAYER_IMPL_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 16 matching lines @@
     : public LayerImpl,
       NON_EXPORTED_BASE(public PictureLayerTilingClient) {
  public:
   class CC_EXPORT LayerRasterTileIterator {
    public:
     LayerRasterTileIterator();
     LayerRasterTileIterator(PictureLayerImpl* layer, bool prioritize_low_res);
     ~LayerRasterTileIterator();
 
     Tile* operator*();
+    const Tile* operator*() const;
     LayerRasterTileIterator& operator++();
     operator bool() const;
 
    private:
     enum IteratorType { LOW_RES, HIGH_RES, NUM_ITERATORS };
 
     PictureLayerImpl* layer_;
 
     struct IterationStage {
       IteratorType iterator_type;
       TilePriority::PriorityBin tile_type;
     };
 
     int current_stage_;
 
     // One low res stage, and three high res stages.
     IterationStage stages_[4];
     PictureLayerTiling::TilingRasterTileIterator iterators_[NUM_ITERATORS];
   };
 
   class CC_EXPORT LayerEvictionTileIterator {
    public:
     LayerEvictionTileIterator();
     LayerEvictionTileIterator(PictureLayerImpl* layer,
                               TreePriority tree_priority);
     ~LayerEvictionTileIterator();
 
     Tile* operator*();
+    const Tile* operator*() const;
     LayerEvictionTileIterator& operator++();
     operator bool() const;
 
    private:
     void AdvanceToNextIterator();
     bool IsCorrectType(
         PictureLayerTiling::TilingEvictionTileIterator* it) const;
 
     std::vector<PictureLayerTiling::TilingEvictionTileIterator> iterators_;
     size_t iterator_index_;
@@ skipping 131 matching lines @@
   gfx::Size viewport_size_for_tile_priority_;
   gfx::Transform screen_space_transform_for_tile_priority_;
 
   friend class PictureLayer;
   DISALLOW_COPY_AND_ASSIGN(PictureLayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_PICTURE_LAYER_IMPL_H_