cc: Add id->LayerImpl map to LayerTreeImpl

cc/layer_impl.h

Index: cc/layer_impl.h
diff --git a/cc/layer_impl.h b/cc/layer_impl.h
index 889e1e11eba85ec828d7711fe586c53fec73b724..4d6e0cd2d11034f943918116a49379b4aac2d498 100644
--- a/cc/layer_impl.h
+++ b/cc/layer_impl.h
@@ -293,6 +293,9 @@ protected:
     static std::string indentString(int indent);
 
 private:
+    scoped_ptr<LayerImpl> takeMaskLayer();
+    scoped_ptr<LayerImpl> takeReplicaLayer();
+
     void setParent(LayerImpl* parent) { m_parent = parent; }
     friend class TreeSynchronizer;
     void clearChildList(); // Warning: This does not preserve tree structure invariants and so is only exposed to the tree synchronizer.