make sure _pageVisible starts out true

fixes self-notifications when the document hasn't gone hidden at least
once since load.

note: i have some code (https://github.com/spladug/visibility) that i'm
working on getting better testing on and eventually want to replace the
visibility shim i'm using here in liveupdate with.  the advantage of
that would be using document.hidden directly and not having to (guess)
that state. oh well.