mus: Add UserActivityMonitor.

components/mus/ws/window_manager_state.h

Index: components/mus/ws/window_manager_state.h
diff --git a/components/mus/ws/window_manager_state.h b/components/mus/ws/window_manager_state.h
index dca6670d400ac522e3b2bc62821babc653840a3f..089114db5706c0a69aaf842bb63b2636a3ccd6d1 100644
--- a/components/mus/ws/window_manager_state.h
+++ b/components/mus/ws/window_manager_state.h
@@ -21,6 +21,7 @@ namespace mus {
 namespace ws {
 
 class DisplayManager;
+class UserActivityMonitor;
 class WindowTree;
 
 namespace test {
@@ -40,6 +41,10 @@ class WindowManagerState : public EventDispatcherDelegate {
   WindowTree* window_tree() { return window_tree_; }
   const WindowTree* window_tree() const { return window_tree_; }
 
+  UserActivityMonitor* user_activity_monitor() {
+    return user_activity_monitor_.get();
+  }
+
   void OnWillDestroyTree(WindowTree* tree);
 
   void SetFrameDecorationValues(mojom::FrameDecorationValuesPtr values);
@@ -166,6 +171,7 @@ class WindowManagerState : public EventDispatcherDelegate {
   base::WeakPtr<Accelerator> post_target_accelerator_;
   std::queue<std::unique_ptr<QueuedEvent>> event_queue_;
   base::OneShotTimer event_ack_timer_;
+  std::unique_ptr<UserActivityMonitor> user_activity_monitor_;
 
   EventDispatcher event_dispatcher_;