LibWeb: Tie layout tree to a specific browsing context

Now that both the layout tree and browsing context are GC-allocated,
we can formalize their relationship a bit better by having layout
nodes keep a NonnullGCPtr to the browsing context.

This makes the previously-indirect link direct, removing an unpleasant
"how do we know the browsing context is alive" question when accessing
it from the layout tree.

Userland/Libraries/LibWeb/Layout/Node.h

@@ -152,6 +152,8 @@ private:
     JS::NonnullGCPtr<DOM::Node> m_dom_node;
     RefPtr<Painting::Paintable> m_paintable;
 
+    JS::NonnullGCPtr<HTML::BrowsingContext> m_browsing_context;
+
     size_t m_serial_id { 0 };
 
     bool m_anonymous { false };