More user id tracking for mus:

components/mus/ws/user_id_tracker_observer.h

Index: components/mus/ws/user_id_tracker_observer.h
diff --git a/components/mus/ws/user_id_tracker_observer.h b/components/mus/ws/user_id_tracker_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..d6b5a86b5285e4f8a81657adf6588995f5430cee
--- /dev/null
+++ b/components/mus/ws/user_id_tracker_observer.h
@@ -0,0 +1,28 @@
+// 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_