Don't clear render surfaces unnecessarily.

cc/trees/layer_tree_host_common.h

 // Copyright 2011 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_HOST_COMMON_H_
 #define CC_TREES_LAYER_TREE_HOST_COMMON_H_
 
 #include <limits>
 #include <vector>
 
@@ skipping 121 matching lines @@
       size_t index) {
     return children[index].get();
   }
 
   static LayerImpl* get_child_as_raw_ptr(
       const OwnedLayerImplList& children,
       size_t index) {
     return children[index];
   }
 
+  static LayerImpl* get_child_as_raw_ptr(const LayerImplList& children,
+                                         size_t index) {
+    return children[index];
+  }
+
   struct ScrollUpdateInfo {
     int layer_id;
     gfx::Vector2d scroll_delta;
   };
 };
 
 struct CC_EXPORT ScrollAndScaleSet {
   ScrollAndScaleSet();
   ~ScrollAndScaleSet();
 
@@ skipping 103 matching lines @@
           true,
           false,
           render_surface_layer_list) {
   DCHECK(root_layer);
   DCHECK(render_surface_layer_list);
 }
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_COMMON_H_