Revert of cc : Make LayerImpl destruction independent of tree hierarchy

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 119 matching lines @@
   void GetAllPrioritizedTilesForTracing(
       std::vector<PrioritizedTile>* prioritized_tiles) const;
   void AsValueInto(base::trace_event::TracedValue* dict) const;
 
   // Other public methods
   // ---------------------------------------------------------------------------
   LayerImpl* root_layer() const { return root_layer_; }
   void SetRootLayer(std::unique_ptr<LayerImpl>);
   bool IsRootLayer(const LayerImpl* layer) const;
   std::unique_ptr<OwnedLayerImplList> DetachLayers();
+  void ClearLayers();
 
   void SetPropertyTrees(PropertyTrees* property_trees);
   PropertyTrees* property_trees() { return &property_trees_; }
 
   void UpdatePropertyTreesForBoundsDelta();
 
   void PushPropertiesTo(LayerTreeImpl* tree_impl);
 
   void MoveChangeTrackingToLayers();
 
@@ skipping 429 matching lines @@
   std::unique_ptr<PendingPageScaleAnimation> pending_page_scale_animation_;
 
  private:
   void BuildLayerListForTesting();
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_IMPL_H_