cc: Remove LayerTreeImpl::root_layer usage outside tests

cc/trees/property_tree.cc

 // Copyright 2014 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 <stddef.h>
 
 #include <set>
 #include <vector>
 
 #include "base/logging.h"
@@ skipping 1999 matching lines @@
     changed |= synced_scroll_offset(key)->PushPendingToActive();
     if (changed) {
       layer_tree_impl->DidUpdateScrollOffset(key, -1);
     }
   }
 }
 
 void ScrollTree::UpdateScrollOffsetMap(
     ScrollTree::ScrollOffsetMap* new_scroll_offset_map,
     LayerTreeImpl* layer_tree_impl) {
-  if (layer_tree_impl && layer_tree_impl->root_layer()) {
+  if (layer_tree_impl && !layer_tree_impl->LayerListIsEmpty()) {
     DCHECK(!property_trees()->is_main_thread);
     for (auto map_entry = layer_id_to_scroll_offset_map_.begin();
          map_entry != layer_id_to_scroll_offset_map_.end();) {
       int key = map_entry->first;
       if (new_scroll_offset_map->find(key) != new_scroll_offset_map->end()) {
         UpdateScrollOffsetMapEntry(key, new_scroll_offset_map, layer_tree_impl);
         ++map_entry;
       } else {
         map_entry = layer_id_to_scroll_offset_map_.erase(map_entry);
       }
@@ skipping 360 matching lines @@
   value->EndDictionary();
 
   value->BeginDictionary("scroll_tree");
   scroll_tree.AsValueInto(value.get());
   value->EndDictionary();
 
   return value;
 }
 
 }  // namespace cc