Don't clear render surfaces unnecessarily.

cc/layers/layer_iterator.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_LAYER_ITERATOR_H_
 #define CC_LAYERS_LAYER_ITERATOR_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/trees/layer_tree_host_common.h"
 
@@ skipping 85 matching lines @@
   bool represents_itself;
   LayerType* target_render_surface_layer;
   LayerType* current_layer;
 };
 
 // An iterator class for walking over layers in the
 // RenderSurface-Layer tree.
 template <typename LayerType>
 class LayerIterator {
   typedef LayerIterator<LayerType> LayerIteratorType;
-  typedef typename LayerType::RenderSurfaceListType LayerList;
+  typedef typename LayerType::LayerListType LayerList;
+  typedef typename LayerType::RenderSurfaceListType RenderSurfaceLayerList;
   typedef typename LayerType::RenderSurfaceType RenderSurfaceType;
 
  public:
   LayerIterator() : render_surface_layer_list_(NULL) {}
 
-  static LayerIteratorType Begin(const LayerList* render_surface_layer_list) {
+  static LayerIteratorType Begin(
+      const RenderSurfaceLayerList* render_surface_layer_list) {
     return LayerIteratorType(render_surface_layer_list, true);
   }
-  static LayerIteratorType End(const LayerList* render_surface_layer_list) {
+  static LayerIteratorType End(
+      const RenderSurfaceLayerList* render_surface_layer_list) {
     return LayerIteratorType(render_surface_layer_list, false);
   }
 
   LayerIteratorType& operator++() {
     MoveToNext();
     return *this;
   }
   bool operator==(const LayerIterator& other) const {
     return target_render_surface_layer_index_ ==
            other.target_render_surface_layer_index_ &&
@@ skipping 28 matching lines @@
         represents_target_render_surface();
     position.represents_contributing_render_surface =
         represents_contributing_render_surface();
     position.represents_itself = represents_itself();
     position.target_render_surface_layer = target_render_surface_layer();
     position.current_layer = current_layer();
     return position;
   }
 
  private:
-  LayerIterator(const LayerList* render_surface_layer_list, bool start)
+  LayerIterator(const RenderSurfaceLayerList* render_surface_layer_list,
+                bool start)
       : render_surface_layer_list_(render_surface_layer_list),
         target_render_surface_layer_index_(0) {
     for (size_t i = 0; i < render_surface_layer_list->size(); ++i) {
       if (!render_surface_layer_list->at(i)->render_surface()) {
         NOTREACHED();
         MoveToEnd();
         return;
       }
     }
 
@@ skipping 64 matching lines @@
       }
       current_layer_index_ = target_render_surface_children().size() - 1;
 
       target_render_surface()->target_render_surface_layer_index_history_ =
           previous_target_render_surface_layer;
     }
   }
 
   inline LayerType* current_layer() const {
     return current_layer_represents_target_render_surface()
-           ? target_render_surface_layer()
-           : target_render_surface_children().at(current_layer_index_);
+               ? target_render_surface_layer()
+               : LayerTreeHostCommon::get_child_as_raw_ptr(
+                     target_render_surface_children(), current_layer_index_);
   }
 
   inline bool current_layer_represents_contributing_render_surface() const {
     return LayerTreeHostCommon::RenderSurfaceContributesToTarget<LayerType>(
         current_layer(), target_render_surface_layer()->id());
   }
   inline bool current_layer_represents_target_render_surface() const {
     return current_layer_index_ ==
            LayerIteratorValue::kLayerIndexRepresentingTargetRenderSurface;
   }
 
   inline RenderSurfaceType* target_render_surface() const {
     return target_render_surface_layer()->render_surface();
   }
   inline const LayerList& target_render_surface_children() const {
     return target_render_surface()->layer_list();
   }
 
-  const LayerList* render_surface_layer_list_;
+  const RenderSurfaceLayerList* render_surface_layer_list_;
 
   // The iterator's current position.
 
   // A position in the render_surface_layer_list. This points to a layer which
   // owns the current target surface. This is a value from 0 to n-1
   // (n = size of render_surface_layer_list = number of surfaces).
   // A value outside of this range
   // (for example, LayerIteratorValue::kInvalidTargetRenderSurfaceLayerIndex)
   // is used to indicate a position outside the bounds of the tree.
   int target_render_surface_layer_index_;
   // A position in the list of layers that are children of the
   // current target surface. When pointing to one of these layers,
   // this is a value from 0 to n-1 (n = number of children).
   // Since the iterator must also stop at the layers representing
   // the target surface, this is done by setting the current_layerIndex
   // to a value of LayerIteratorValue::LayerRepresentingTargetRenderSurface.
   int current_layer_index_;
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_ITERATOR_H_