mus: Add UserActivityMonitor.

components/mus/ws/window_server.h

Index: components/mus/ws/window_server.h
diff --git a/components/mus/ws/window_server.h b/components/mus/ws/window_server.h
index d8e463d0207406044491675a93519a38891b51fe..598899f1cff0ed46384cbd55420818b74adbd92d 100644
--- a/components/mus/ws/window_server.h
+++ b/components/mus/ws/window_server.h
@@ -34,6 +34,7 @@ namespace ws {
 class AccessPolicy;
 class DisplayManager;
 class ServerWindow;
+class UserActivityMonitor;
 class WindowManagerState;
 class WindowServerDelegate;
 class WindowTree;
@@ -127,6 +128,8 @@ class WindowServer : public ServerWindowDelegate,
 
   void OnFirstWindowManagerWindowTreeFactoryReady();
 
+  UserActivityMonitor* GetUserActivityMonitorForUser(const UserId& user_id);
+
   WindowManagerWindowTreeFactorySet* window_manager_window_tree_factory_set() {
     return &window_manager_window_tree_factory_set_;
   }