The invariants for these property getters are supposed to be checked by
the has_entries_and_events_disabled AO, but we don't have all the
plumbing hooked up between Navigables and Navigation yet.
Add a test to make sure that these methods don't assert when calling
them on a fresh page.
The implementation is incomplete, because our Navigable::navigate
implementation is missing the navigationAPIState parameter. We also
don't have Navigables hooked up completely enough to guarantee that a
fully active document that is not being unloaded always has a Navigable.
This API is how JavaScript can manipulate the new Navigable concepts
directly. We are still missing most of the interesting algorithms on
Navigation that do the actual navigation steps, and call into the
currently WIP navigable AOs.
