Implements basic form resetting

What can this do? Reset `<input type=text>` fields back to their default
value through a call to a form's reset method. That's all for now!

Fixes compile error after rebase

components/script/dom/webidls/HTMLFormElement.webidl

@@ -22,7 +22,7 @@ interface HTMLFormElement : HTMLElement {
   //getter (RadioNodeList or Element) (DOMString name);
 
   void submit();
-  //void reset();
+  void reset();
   //boolean checkValidity();
   //boolean reportValidity();