More user id tracking for mus:

components/mus/ws/user_id_tracker.h

Index: components/mus/ws/user_id_tracker.h
diff --git a/components/mus/ws/user_id_tracker.h b/components/mus/ws/user_id_tracker.h
new file mode 100644
index 0000000000000000000000000000000000000000..5a97e6e0c59d7a61e32c8ad62437cccf9a31809b
--- /dev/null
+++ b/components/mus/ws/user_id_tracker.h
@@ -0,0 +1,49 @@
+// 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_H_
+#define COMPONENTS_MUS_WS_USER_ID_TRACKER_H_
+
+#include <stdint.h>
+
+#include <set>
+
+#include "base/macros.h"
+#include "base/observer_list.h"
+#include "components/mus/ws/user_id.h"
+
+namespace mus {
+namespace ws {
+
+class UserIdTrackerObserver;
+
+// Tracks the set of known/valid user ids.
+class UserIdTracker {
+ public:
+  UserIdTracker();
+  ~UserIdTracker();
+
+  bool IsValidUserId(UserId id) const;
+
+  void SetActiveUserId(UserId id);
+  void AddUserId(UserId id);
+  void RemoveUserId(UserId id);
+
+  void AddObserver(UserIdTrackerObserver* observer);
+  void RemoveObserver(UserIdTrackerObserver* observer);
+
+ private:
+  using UserIdSet = std::set<UserId>;
+
+  base::ObserverList<UserIdTrackerObserver> observers_;
+  UserIdSet ids_;
+  UserId active_id_;
+
+  DISALLOW_COPY_AND_ASSIGN(UserIdTracker);
+};
+
+}  // namespace ws
+}  // namespace mus
+
+#endif  // COMPONENTS_MUS_WS_USER_ID_TRACKER_H_