[PageVisibility] location of the interface

The NavigatorVisibility interface is defined to be implemented by the
Navigator object.  I don’t think this is the best place, since the
Navigator object has traditionally been a location for browser-wide
information and functionality to be exposed.  For example, see the kinds
of things HTML5 defines on window.navigator:

  http://www.whatwg.org/specs/web-apps/current-work/multipage/timers.html#system-state-and-capabilities:-the-navigator-object

I think this would be better either on the Window object or on the
Document object.

-- 
Cameron McCormack ≝ http://mcc.id.au/