cc: Remove LayerTreeImpl::root_layer usage outside tests

cc/trees/tree_synchronizer.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/tree_synchronizer.h"
 
 #include <stddef.h>
 
 #include <set>
 
@@ skipping 32 matching lines @@
 void TreeSynchronizer::SynchronizeTrees(Layer* layer_root,
                                         LayerTreeImpl* tree_impl) {
   if (!layer_root) {
     tree_impl->DetachLayers();
   } else {
     SynchronizeTreesInternal(layer_root->layer_tree_host(), tree_impl,
                              layer_root->layer_tree_host()->property_trees());
   }
 }
 
-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());
   }
 }
 
 template <typename LayerType>
 std::unique_ptr<LayerImpl> ReuseOrCreateLayerImpl(OwnedLayerImplMap* old_layers,
                                                   LayerType* layer,
                                                   LayerTreeImpl* tree_impl) {
   if (!layer)
     return nullptr;
   std::unique_ptr<LayerImpl> layer_impl = std::move((*old_layers)[layer->id()]);
@@ skipping 34 matching lines @@
                               active_tree);
 }
 
 void TreeSynchronizer::PushLayerProperties(LayerTreeHost* host_tree,
                                            LayerTreeImpl* impl_tree) {
   PushLayerPropertiesInternal(host_tree->LayersThatShouldPushProperties(),
                               impl_tree);
 }
 
 }  // namespace cc