cc: Remove LayerTreeImpl::root_layer usage outside tests

cc/trees/tree_synchronizer.h

Index: cc/trees/tree_synchronizer.h
diff --git a/cc/trees/tree_synchronizer.h b/cc/trees/tree_synchronizer.h
index be2e7439e772607be29096a9e6cb60a59e8b2cc1..e5a0b62a0f67e050cff10364bc763a17b0aee96f 100644
--- a/cc/trees/tree_synchronizer.h
+++ b/cc/trees/tree_synchronizer.h
@@ -23,7 +23,8 @@ class CC_EXPORT TreeSynchronizer {
   // duplicates the structure of the Layer tree, reusing the LayerImpls in the
   // tree provided by old_layer_impl_root if possible.
   static void SynchronizeTrees(Layer* layer_root, LayerTreeImpl* tree_impl);
-  static void SynchronizeTrees(LayerImpl* layer_root, LayerTreeImpl* tree_impl);
+  static void SynchronizeTrees(LayerTreeImpl* pending_tree,
+                               LayerTreeImpl* active_tree);
 
   static void PushLayerProperties(LayerTreeImpl* pending_tree,
                                   LayerTreeImpl* active_tree);