Add SessionStateObserver with ActiveUserChanged()

ash/session_state_delegate.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_DELEGATE_H_
 #define ASH_SESSION_STATE_DELEGATE_H_
 
 #include <string>
 #include <vector>
 
 #include "ash/ash_export.h"
+#include "ash/session_state_observer.h"
 #include "base/string16.h"
 
 namespace gfx {
 class ImageSkia;
 }  // namespace gfx
 
 namespace ash {
 
 // The index for the multi-profile item to use. The list is always LRU sorted
 // So that the index #0 is the currently active user.
@@ skipping 43 matching lines @@
 
   // Gets the avatar image for the user with the given |index|.
   // Note that |index| can at maximum be |NumberOfLoggedInUsers() - 1|.
   virtual const gfx::ImageSkia& GetUserImage(MultiProfileIndex index) const = 0;
 
   // Returns a list of all logged in users.
   virtual void GetLoggedInUsers(UserEmailList* users) = 0;
 
   // Switches to another active user (if that user has already signed in).
   virtual void SwitchActiveUser(const std::string& email) = 0;
+
+  // Adds or removes sessions state observer.
+  virtual void AddSessionStateObserver(SessionStateObserver* observer) = 0;
+  virtual void RemoveSessionStateObserver(SessionStateObserver* observer) = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_SESSION_STATE_DELEGATE_H_