fix: don't assign NSAlert to window which is not visible

Without this change it's possible to create message box which can't
be dismissed on mac.

shell/browser/ui/message_box_mac.mm

@@ -93,12 +93,11 @@
 
 }  // namespace
 
-int ShowMessageBoxSync(const MessageBoxSettings& settings) {
   NSAlert* alert = CreateNSAlert(settings);
 
   // Use runModal for synchronous alert without parent, since we don't have a
   // window to wait for.
-  if (!settings.parent_window)
+  if (!settings.parent_window || !settings.parent_window->IsVisible())
     return [[alert autorelease] runModal];
 
   __block int ret_code = -1;