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) {}

[sky, 2013/06/03 15:06:23]

How come this is in terms of user_id but the other methods in
SessionStateDelegate are in terms of an index and or an email address? Can we
have some consistency here?

I didn't review the changes to SessionStateDelegate, but looking at them
wouldn't it make more sense to have a User and a method that returns a
vector<User> rather than all the other methods that were exposed? Also,
SessionState seems to have two very distinct sets of methods, one for user state
and the other for lock state. Should they be separated out?

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