LibWeb: Implement [SameObject] behavior for HTMLFormElement.elements

Author: https://github.com/awesomekling
Commit: d7c58aa58d
Pull-request: https://github.com/SerenityOS/serenity/pull/16175
Reviewed-by: https://github.com/ADKaster

Userland/Libraries/LibWeb/HTML/HTMLFormElement.h

@@ -39,6 +39,8 @@ private:
     bool m_firing_submission_events { false };
 
     Vector<JS::GCPtr<HTMLElement>> m_associated_elements;
+
+    JS::GCPtr<DOM::HTMLCollection> mutable m_elements;
 };
 
 }