LibWeb: Add GC finalizer for DOM::Node

Now that the layout tree is also GC-allocated, we can't be messing with
it from the DOM::Node destructor. Move everything to a GC finalizer
so we know it runs before the GC sweep phase.

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -73,7 +73,9 @@ Node::Node(Document& document, NodeType type)
 {
 }
 
-Node::~Node()
+Node::~Node() = default;
+
+void Node::finalize()
 {
     if (layout_node() && layout_node()->parent())
         layout_node()->parent()->remove_child(*layout_node());

Userland/Libraries/LibWeb/DOM/Node.h

@@ -622,6 +622,7 @@ protected:
     Node(Document&, NodeType);
 
     virtual void visit_edges(Cell::Visitor&) override;
+    virtual void finalize() override;
 
     JS::GCPtr<Document> m_document;
     JS::GCPtr<Layout::Node> m_layout_node;