Auto merge of #11697 - mrmiywj:form-index-getter, r=Ms2ger

indexed getter of form elements

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- [X] These changes fix #11405 (github issue number if applicable).

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components/script/dom/webidls/HTMLFormElement.webidl

@@ -17,7 +17,7 @@ interface HTMLFormElement : HTMLElement {
 
   [SameObject] readonly attribute HTMLFormControlsCollection elements;
   readonly attribute unsigned long length;
-  //getter Element (unsigned long index);
+  getter Element? (unsigned long index);
   //getter (RadioNodeList or Element) (DOMString name);
 
   void submit();