session_manager: Add SessionManagerObserver interface

components/session_manager/core/session_manager_observer.h

+// Copyright 2016 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 COMPONENTS_SESSION_MANAGER_CORE_SESSION_MANAGER_OBSERVER_H_
+#define COMPONENTS_SESSION_MANAGER_CORE_SESSION_MANAGER_OBSERVER_H_
+
+#include "base/macros.h"
+#include "components/session_manager/session_manager_types.h"
+
+namespace session_manager {
+
+// An observer interface for SessionManager.
+// TODO(xiyuan): Use this to replace UserManager::UserSessionStateObserver,
+//     http://crbug.com/657149.
+class SessionManagerObserver {
+ public:
+  SessionManagerObserver() = default;
+
+  // Invoked when session state is changed.
+  virtual void OnSessionStateChanged() = 0;
+
+ protected:
+  virtual ~SessionManagerObserver() = default;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(SessionManagerObserver);
+};
+
+}  // namespace session_manager
+
+#endif  // COMPONENTS_SESSION_MANAGER_CORE_SESSION_MANAGER_OBSERVER_H_