cc: Make LayerTreeImpl dump layer list to FrameViewer and Devtools

cc/trees/layer_tree_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_TREES_LAYER_TREE_IMPL_H_
 #define CC_TREES_LAYER_TREE_IMPL_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 140 matching lines @@
       property_trees_.effect_tree.set_needs_update(true);
   }
   PropertyTrees* property_trees() { return &property_trees_; }
 
   void UpdatePropertyTreesForBoundsDelta();
 
   void PushPropertiesTo(LayerTreeImpl* tree_impl);
 
   void MoveChangeTrackingToLayers();
 
-  LayerListIterator<LayerImpl> begin();
-  LayerListIterator<LayerImpl> end();
+  LayerListIterator<LayerImpl> begin() const;
+  LayerListIterator<LayerImpl> end() const;
   LayerListReverseIterator<LayerImpl> rbegin();
   LayerListReverseIterator<LayerImpl> rend();
 
   struct CC_EXPORT ElementLayers {
     // Transform and opacity mutations apply to this layer.
     LayerImpl* main = nullptr;
     // Scroll mutations apply to this layer.
     LayerImpl* scroll = nullptr;
   };
 
@@ skipping 413 matching lines @@
 
   std::unique_ptr<PendingPageScaleAnimation> pending_page_scale_animation_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_IMPL_H_