Revert of cc: Remove use of PassAs() and constructor-casting with scoped_ptr.

cc/trees/tree_synchronizer.cc

Index: cc/trees/tree_synchronizer.cc
diff --git a/cc/trees/tree_synchronizer.cc b/cc/trees/tree_synchronizer.cc
index d17f0175b9d9cbd10491b57ba632814760366246..1faf0e04e679d468b91ab49ef5e209794d8abea5 100644
--- a/cc/trees/tree_synchronizer.cc
+++ b/cc/trees/tree_synchronizer.cc
@@ -105,7 +105,7 @@
     LayerType* layer,
     LayerTreeImpl* tree_impl) {
   if (!layer)
-    return nullptr;
+    return scoped_ptr<LayerImpl>();
 
   scoped_ptr<LayerImpl> layer_impl =
       ReuseOrCreateLayerImpl(new_layers, old_layers, layer, tree_impl);