Replace more ObserverList with base::ObserverList.

ash/system/user/user_accounts_delegate.h

 // 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.
 
 #ifndef ASH_SYSTEM_USER_USER_ACCOUNTS_DELEGATE_H_
 #define ASH_SYSTEM_USER_USER_ACCOUNTS_DELEGATE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 37 matching lines @@
   // |account_id| that is not from list is no-op.
   virtual void DeleteAccount(const std::string& account_id) = 0;
 
   // Launches a dialog which lets the user add a new account.
   virtual void LaunchAddAccountDialog() = 0;
 
  protected:
   void NotifyAccountListChanged();
 
  private:
-  ObserverList<Observer> observers_;
+  base::ObserverList<Observer> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(UserAccountsDelegate);
 };
 
 }  // namespace tray
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_USER_USER_ACCOUNTS_DELEGATE_H_