cc: Remove LayerTreeImpl::root_layer usage outside tests

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 4069a3437075c6214a7831b398dc3b2b45962ef9..ee02df7f9246700d39b5553b83af8a6b9b0fffab 100644
--- a/cc/trees/tree_synchronizer.cc
+++ b/cc/trees/tree_synchronizer.cc
@@ -50,13 +50,13 @@ void TreeSynchronizer::SynchronizeTrees(Layer* layer_root,
   }
 }
 
-void TreeSynchronizer::SynchronizeTrees(LayerImpl* layer_root,
-                                        LayerTreeImpl* tree_impl) {
-  if (!layer_root) {
-    tree_impl->DetachLayers();
+void TreeSynchronizer::SynchronizeTrees(LayerTreeImpl* pending_tree,
+                                        LayerTreeImpl* active_tree) {
+  if (pending_tree->LayerListIsEmpty()) {
+    active_tree->DetachLayers();
   } else {
-    SynchronizeTreesInternal(layer_root->layer_tree_impl(), tree_impl,
-                             layer_root->layer_tree_impl()->property_trees());
+    SynchronizeTreesInternal(pending_tree, active_tree,
+                             pending_tree->property_trees());
   }
 }