More user id tracking for mus:

components/mus/ws/user_id_tracker_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_MUS_WS_USER_ID_TRACKER_OBSERVER_H_
+#define COMPONENTS_MUS_WS_USER_ID_TRACKER_OBSERVER_H_
+
+#include <stdint.h>
+
+#include "components/mus/ws/user_id.h"
+
+namespace mus {
+namespace ws {
+
+class UserIdTrackerObserver {
+ public:
+  virtual void OnActiveUserIdChanged(UserId id) = 0;
+  virtual void OnUserIdAdded(UserId id) = 0;
+  virtual void OnUserIdRemoved(UserId id) = 0;
+
+ protected:
+  virtual ~UserIdTrackerObserver() {}
+};
+
+}  // namespace ws
+}  // namespace mus
+
+#endif  // COMPONENTS_MUS_WS_USER_ID_TRACKER_OBSERVER_H_