Allow detection of non-intercepted ongoing navigations

[tbondwilkinson]

For certain navigations like cross-document navigations, we have no ability to detect that they are occurring unless they are intercepted. It would be nice to additionally expose them as part of navigation.transition.

The workaround is to set a boolean that the cross-document navigation is occurring, but this requires also unsetting the bit on pageshow because we have no way to observe when the cross-document navigation has finished otherwise. Instead, checking navigation.transition would allow us to only reject traversals occurring a cross-document navigation