More user id tracking for mus:

components/mus/ws/window_manager_factory_registry_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_WINDOW_MANAGER_FACTORY_REGISTRY_OBSERVER_H_
+#define COMPONENTS_MUS_WS_WINDOW_MANAGER_FACTORY_REGISTRY_OBSERVER_H_
+
+namespace mus {
+namespace ws {
+
+class WindowManagerFactoryService;
+
+class WindowManagerFactoryRegistryObserver {
+ public:
+  // Called when the mojom::WindowManagerFactory associated with |service| has
+  // been obtained.
+  virtual void OnWindowManagerFactorySet(
+      WindowManagerFactoryService* service) = 0;
+
+ protected:
+  virtual ~WindowManagerFactoryRegistryObserver() {}
+};
+
+}  // namespace ws
+}  // namespace mus
+
+#endif  // COMPONENTS_MUS_WS_WINDOW_MANAGER_FACTORY_REGISTRY_OBSERVER_H_