[Spec] Spec that disabling untrusted network disables top-level navigations (_unfencedTop and popups)

[gtanzer]

Do something similar to #148 but for _unfencedTop and popups

[gtanzer]

@domfarolino At least the _unfencedTop part of this, you said there were parts of the spec that had been missed before and that you would fix.

[domfarolino]

Then that should probably be a separate issue (i.e., spec'ing _unfencedTop itself). That seems unrelated to the meat of this issue, although I agree this issue would be blocked on the new one that should be assigned to me.

[gtanzer]

Done and assigned you there rather than here

[domfarolino]

I know I mentioned it should be assigned to me, but just want to mention it is unlikely I'd be able to get to it for a while, so it might be worth assigning to someone more active on the project in case they can get to it before I would in the future. (I'm basically saying I should be a last resort assignment for that one).