Consuming the new 'page-visited' notification was fairly trivial,
since it was already brought over to onVisits. There's not much to
say about this other than that I'm a little bit uncertain about
all the hoops we have to jump through to get a JSContext and
GlobalObject from History.cpp (which is discussed in the earlier
commit in the series).
MozReview-Commit-ID: LHaBWSylyLI
cc2403e3ed