[Sync] Replace typedef with using.

components/sync/base/extensions_activity.h

 // Copyright 2013 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_EXTENSIONS_ACTIVITY_H_
 #define COMPONENTS_SYNC_BASE_EXTENSIONS_ACTIVITY_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 17 matching lines @@
 
     // The human-readable ID identifying the extension responsible
     // for the activity reported in this Record.
     std::string extension_id;
 
     // How many times the extension successfully invoked a write
     // operation through the bookmarks API since the last CommitMessage.
     uint32_t bookmark_write_count;
   };
 
-  typedef std::map<std::string, Record> Records;
+  using Records = std::map<std::string, Record>;
 
   ExtensionsActivity();
 
   // Fill |buffer| with all current records and then clear the
   // internal records. Called on sync thread to append records to sync commit
   // message.
   void GetAndClearRecords(Records* buffer);
 
   // Merge |records| with the current set of records. Called on sync thread to
   // put back records if sync commit failed.
   void PutRecords(const Records& records);
 
   // Increment write count of the specified extension.
   void UpdateRecord(const std::string& extension_id);
 
  private:
   friend class base::RefCountedThreadSafe<ExtensionsActivity>;
   ~ExtensionsActivity();
 
   Records records_;
   mutable base::Lock records_lock_;
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_BASE_EXTENSIONS_ACTIVITY_H_