Activate LayerImpl tree with sync+push instead of pointer swap

cc/layer.cc

Index: cc/layer.cc
diff --git a/cc/layer.cc b/cc/layer.cc
index a742d073f81de9864111049949510e06b4554998..7a263aaa6589db9db42813ab7e581b2fa0c6c40c 100644
--- a/cc/layer.cc
+++ b/cc/layer.cc
@@ -284,6 +284,12 @@ void Layer::setChildren(const LayerList& children)
         addChild(children[i]);
 }
 
+Layer* Layer::childAt(size_t index) const
+{
+  DCHECK_LT(index, m_children.size());
+  return m_children[index].get();
+}
+
 void Layer::setAnchorPoint(const gfx::PointF& anchorPoint)
 {
     if (m_anchorPoint == anchorPoint)