LibWeb: Add a visit_edges() override to AbstractBrowsingContext

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.

Userland/Libraries/LibWeb/HTML/AbstractBrowsingContext.h

@@ -34,6 +34,8 @@ public:
     virtual void set_window_handle(String handle) = 0;
 
 protected:
+    virtual void visit_edges(JS::Cell::Visitor&) override;
+
     String m_name;
 
     // https://html.spec.whatwg.org/multipage/browsers.html#is-popup