Bug 1787684. Only return views with widgets from FindFloatingViewContaining. r=emilio, a=RyanVM

The code that bug 1754436 deleted checked to make sure there was a widget for the returned view, but it looks like it's possible in some edge cases to have an open popup that doesn't have a widget (most likely just for a split second). We need a widget to dispatch the event, and if it doesn't have a widget it's not visible so no point in touching it at all anyways.

Differential Revision: https://phabricator.services.mozilla.com/D155819