Support MenuWidget's config.widget being in a different document

MenuWidget has a nice feature where you can specify another widget
for its mousedown-on-close handling, but it only listens for mousedown
in its own document. If the other widget is in a different document,
we need to listen to mousedown in that document too.

Bug: 72099
Change-Id: If3e73ed0d34610e8512ad49c35661c3d3cfd7b75