LibWeb: Add GC finalizer to DOM::NodeIterator

It's potentially unsafe to access `m_root` in the destructor since it
may have been swept, so move unregistration of the NodeIterator into a
GC finalizer instead.

Userland/Libraries/LibWeb/DOM/NodeIterator.h

@@ -38,6 +38,7 @@ private:
     explicit NodeIterator(Node& root);
 
     virtual void visit_edges(Cell::Visitor&) override;
+    virtual void finalize() override;
 
     enum class Direction {
         Next,