We should use UserID object to identify users instead of username.

ash/session/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_SESSION_STATE_OBSERVER_H_
 #define ASH_SESSION_SESSION_STATE_OBSERVER_H_
 
 #include <string>
 
 #include "ash/ash_export.h"
 #include "ash/session/session_state_delegate.h"
 #include "base/basictypes.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace ash {
 
 class ASH_EXPORT SessionStateObserver {
  public:
   // Called when active user has changed.
-  virtual void ActiveUserChanged(const std::string& user_id) {}
+  virtual void ActiveUserChanged(const user_manager::UserID& user_id) {}
 
   // Called when another user gets added to the existing session.
-  virtual void UserAddedToSession(const std::string& user_id) {}
+  virtual void UserAddedToSession(const user_manager::UserID& user_id) {}
 
   // Called when session state is changed.
   virtual void SessionStateChanged(SessionStateDelegate::SessionState state) {}
 
  protected:
   virtual ~SessionStateObserver() {}
 };
 
 // A class to attach / detach an object as a session state observer with a
 // scoped pointer.
 class ASH_EXPORT ScopedSessionStateObserver {
  public:
   explicit ScopedSessionStateObserver(ash::SessionStateObserver* observer);
   virtual ~ScopedSessionStateObserver();
 
  private:
   ash::SessionStateObserver* observer_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedSessionStateObserver);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SESSION_SESSION_STATE_OBSERVER_H_