Adding "UserAddedToSession" notifier to the SessionStateObserver

ash/session_state_observer.h

 // Copyright (c) 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 ASH_SESSION_STATE_OBSERVER_H_
 #define ASH_SESSION_STATE_OBSERVER_H_
 
 #include <string>
 
 #include "ash/ash_export.h"
 
 namespace ash {
 
 class ASH_EXPORT SessionStateObserver {
  public:
   // Called when active user has changed.
   virtual void ActiveUserChanged(const std::string& user_id) {}
 
+  // Called when another user gets added to the existing session.
+  virtual void UserAddedToSession(const std::string& user_id) {}
+
  protected:
   virtual ~SessionStateObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_SESSION_STATE_OBSERVER_H_