Wires up transients for DesktopWindowTreeHostMus

ui/views/mus/desktop_window_tree_host_mus.h

Index: ui/views/mus/desktop_window_tree_host_mus.h
diff --git a/ui/views/mus/desktop_window_tree_host_mus.h b/ui/views/mus/desktop_window_tree_host_mus.h
index da57ad174e76601cf00d789aa0da146344389244..16ad8034a222dd8492365a0c77048cdd4354dee2 100644
--- a/ui/views/mus/desktop_window_tree_host_mus.h
+++ b/ui/views/mus/desktop_window_tree_host_mus.h
@@ -95,6 +95,11 @@ class VIEWS_MUS_EXPORT DesktopWindowTreeHostMus
   // fullscreen.
   ui::WindowShowState fullscreen_restore_state_;
 
+  // We can optionally have a parent which can order us to close, or own
+  // children who we're responsible for closing when we CloseNow().
+  DesktopWindowTreeHostMus* parent_ = nullptr;
+  std::set<DesktopWindowTreeHostMus*> children_;
+
   // Used so that Close() isn't immediate.
   base::WeakPtrFactory<DesktopWindowTreeHostMus> close_widget_factory_;