[Sync] Replace typedef with using.

components/sync/base/sync_prefs.h

 // Copyright (c) 2012 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.
 
 #ifndef COMPONENTS_SYNC_BASE_SYNC_PREFS_H_
 #define COMPONENTS_SYNC_BASE_SYNC_PREFS_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 199 matching lines @@
   // configuration management.
   BooleanPrefMember pref_sync_managed_;
 
   // Groups of prefs that always have the same value as a "master" pref.
   // For example, the APPS group has {APP_NOTIFICATIONS, APP_SETTINGS}
   // (as well as APPS, but that is implied), so
   //   pref_groups_[APPS] =       { APP_NOTIFICATIONS,
   //                                          APP_SETTINGS }
   //   pref_groups_[EXTENSIONS] = { EXTENSION_SETTINGS }
   // etc.
-  typedef std::map<ModelType, ModelTypeSet> PrefGroupsMap;
+  using PrefGroupsMap = std::map<ModelType, ModelTypeSet>;
   PrefGroupsMap pref_groups_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncPrefs);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_BASE_SYNC_PREFS_H_