Reland: mus: Enable system modal windows

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 d85199b5f4be81f06963dc6299e29f18a8050499..c8edfebb91a79fda8c2cf2e6ba9471fb7fbef437 100644
--- a/components/mus/ws/window_manager_state.h
+++ b/components/mus/ws/window_manager_state.h
@@ -82,6 +82,14 @@ class WindowManagerState : public EventDispatcherDelegate {
   // and if that's the case, releases the capture.
   void ReleaseCaptureBlockedByAnyModalWindow();
 
+  bool SetSystemModalWindow(ServerWindow* window);
+  ServerWindow* system_modal_window() {
+    return event_dispatcher_.system_modal_window();
+  }
+  const ServerWindow* system_modal_window() const {
+    return event_dispatcher_.system_modal_window();
+  }
+
   // Returns true if this is the WindowManager of the active user.
   bool IsActive() const;