Activate LayerImpl tree with sync+push instead of pointer swap

cc/layer.h

Index: cc/layer.h
diff --git a/cc/layer.h b/cc/layer.h
index a510ab60e00df897e0189bb3b931bd626fcc18af..0c1670fa7dfa788b4136e4241e1f04c14a31d150 100644
--- a/cc/layer.h
+++ b/cc/layer.h
@@ -67,6 +67,7 @@ public:
     void setChildren(const LayerList&);
 
     const LayerList& children() const { return m_children; }
+    Layer* childAt(size_t index);
 
     void setAnchorPoint(const gfx::PointF&);
     gfx::PointF anchorPoint() const { return m_anchorPoint; }
@@ -291,6 +292,9 @@ public:
 
     virtual bool canClipSelf() const;
 
+    // Constructs a LayerImpl of the correct runtime type for this Layer type.
+    virtual scoped_ptr<LayerImpl> createLayerImpl(LayerTreeImpl* treeImpl);
+
 protected:
     friend class LayerImpl;
     friend class TreeSynchronizer;
@@ -315,8 +319,6 @@ protected:
 
     scoped_refptr<Layer> m_maskLayer;
 
-    // Constructs a LayerImpl of the correct runtime type for this Layer type.
-    virtual scoped_ptr<LayerImpl> createLayerImpl(LayerTreeImpl* treeImpl);
     int m_layerId;
 
     // When true, the layer is about to perform an update. Any commit requests