LibWeb: Implement more of "completely finish loading the document"

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/5f4a723e515

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.h

@@ -26,4 +26,6 @@ private:
     void load_src(const String&);
 };
 
+void run_iframe_load_event_steps(HTML::HTMLIFrameElement&);
+
 }