AbstractBrowsingContext has a subclass RemoteBrowsingContext without a
visit_edges() override (and it doesn't really need one). But currently,
we rely on subclasses visiting AbstractBrowsingContext's opener BC.
This adds a visit_edges() to AbstractBrowsingContext to explicitly visit
the opener BC itself.
Introducing class for abstract browsing context is going to make it
possible to have separate implementations for:
1) Local browsing context (Top level BC of current page and iframes)
2) Remote browsing context (BC of a page that lives in another
WebContent process)
