popstate transitioning between documents may not be interoperable

[domenic]

Porting from https://www.w3.org/Bugs/Public/show_bug.cgi?id=27188. This investigated scenarios like

navigating multiple steps backwards or forwards in history (e.g. history.go(-2)) between different documents. This lets you navigate to a session entry that's not its document's "latest entry", in which case, per current specification, a popstate event should be fired in addition to the pageshow that is fired.

The conclusion there seemed to be that the current spec for when to fire popstate is roughly correct and that there may be browser bugs.

(Note that popstate has some other issues e.g. #1042 and #1792.)

We should write web platform tests covering those cases and confirm the spec is reasonable. Ideally it would match at least some browsers; if not we may want to reconsider the tradeoff between what browsers want (expressed in that bug to be equal to the current spec) and what they actually do.