Add SessionStateObserver with ActiveUserChanged()

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) {}
+
+ protected:
+  virtual ~SessionStateObserver() {}
+};
+
+}  // namespace ash
+
+#endif  // ASH_SESSION_STATE_OBSERVER_H_