Check if we use "Browsing Context" in our specs

[alvestrand]

With the rewrite done in whatwg/html#6315 it seems that HTML5 has abandoned the concept of "browsing context", introducing instead "navigable" and more.

We should check if we use that concept anywhere.

[dontcallmedom]

we use "nested browsing context", which has disappeared from the HTML spec (and thus generates a ReSpec error at the moment); if I read whatwg/html#8462 correctly, it looks like it should be replaced with "child navigable" cc @jan-ivar

[jan-ivar]

I fixed the ReSpec error in #912 to unblock merge.

I count 11 other uses of "browsing context" though, that I think need updating. It seems we mean navigable sometimes, and other times Document.