LibWeb/HTML: Stop observing lazy-loaded iframes when navigating

Author: https://github.com/AtkinsSJ
Commit: f11347739a
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3171

Libraries/LibWeb/HTML/HTMLIFrameElement.h

@@ -40,6 +40,9 @@ private:
 
     virtual void initialize(JS::Realm&) override;
 
+    // ^DOM::Node
+    virtual bool is_html_iframe_element() const override { return true; }
+
     // ^DOM::Element
     virtual void post_connection() override;
     virtual void removed_from(Node*) override;