Check if parent_window exists before clearing transient state.

It could happen that the dialog window is closed because the parent
window was closed (e.g. Ctrl+C on Chrome running on terminal). In that
case, the parent X11Window object doesn't exist any more, and calling
it would cause a segfault.

Bug: 1104585
Change-Id: I3aee31c5fa9e567f55e0317d9df8c675502ff990
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300094Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nick Yamane <nickdiego@igalia.com>
Commit-Queue: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Cr-Commit-Position: refs/heads/master@{#789436}

ui/gtk/x/gtk_ui_delegate_x11.cc

@@ -63,7 +63,9 @@ bool GtkUiDelegateX11::SetGdkWindowTransientFor(GdkWindow* window,
 void GtkUiDelegateX11::ClearTransientFor(gfx::AcceleratedWidget parent) {
   ui::X11Window* parent_window =
       ui::X11WindowManager::GetInstance()->GetWindow(parent);
-  parent_window->SetTransientWindow(x11::Window::None);
+  // parent_window might be dead if there was a top-down window close
+  if (parent_window)
+    parent_window->SetTransientWindow(x11::Window::None);
 }
 
 GdkDisplay* GtkUiDelegateX11::GetGdkDisplay() {