Activate LayerImpl tree with sync+push instead of pointer swap

cc/layer.cc

 // Copyright 2010 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.
 
 #include "cc/layer.h"
 
 #include "cc/animation.h"
 #include "cc/animation_events.h"
 #include "cc/layer_animation_controller.h"
 #include "cc/layer_impl.h"
@@ skipping 266 matching lines @@
 {
     if (children == m_children)
         return;
 
     removeAllChildren();
     size_t listSize = children.size();
     for (size_t i = 0; i < listSize; i++)
         addChild(children[i]);
 }
 
+Layer* Layer::childAt(size_t index)
+{
+  DCHECK_LT(index, m_children.size());
+  return m_children[index].get();
+}
+
 void Layer::setAnchorPoint(const gfx::PointF& anchorPoint)
 {
     if (m_anchorPoint == anchorPoint)
         return;
     m_anchorPoint = anchorPoint;
     setNeedsCommit();
 }
 
 void Layer::setAnchorPointZ(float anchorPointZ)
 {
@@ skipping 569 matching lines @@
 {
     return 0;
 }
 
 void sortLayers(std::vector<scoped_refptr<Layer> >::iterator, std::vector<scoped_refptr<Layer> >::iterator, void*)
 {
     // Currently we don't use z-order to decide what to paint, so there's no need to actually sort Layers.
 }
 
 }  // namespace cc