Core: Treat overflow:hidden as scrollable in $.ui.scrollParent

Even though the user is unable to scroll via the UI, authors
may have custom scrollbars that programmatically set scrollTop.
Therefore, overflow:hidden must be considered scrollable.
It seems a little odd to consider an `overflow: hidden` element as a “scrollParent”, but I think I get why you're doing this here. I worry a bit about this having adverse in other cases.