cc: Add some more infrastructure for two trees

cc/layer_tree_host_impl.h

Index: cc/layer_tree_host_impl.h
diff --git a/cc/layer_tree_host_impl.h b/cc/layer_tree_host_impl.h
index 9943da20e7d4b1a8e663b2b9e40199ad9e656f17..79c26c02fb5ae982552c968edf152cfa2d758678 100644
--- a/cc/layer_tree_host_impl.h
+++ b/cc/layer_tree_host_impl.h
@@ -204,6 +204,7 @@ public:
     void readback(void* pixels, const gfx::Rect&);
 
     LayerTreeImpl* activeTree() { return m_activeTree.get(); }
+    LayerTreeImpl* pendingTree() { return m_pendingTree.get(); }
 
     // TODO(nduca): Remove these in favor of LayerTreeImpl.
     void setRootLayer(scoped_ptr<LayerImpl>);
@@ -354,6 +355,7 @@ private:
     scoped_ptr<Renderer> m_renderer;
     scoped_ptr<TileManager> m_tileManager;
 
+    scoped_ptr<LayerTreeImpl> m_pendingTree;
     scoped_ptr<LayerTreeImpl> m_activeTree;
 
     bool m_scrollDeltaIsInViewportSpace;