Implement HTMLElement.dataset

Make DOMStringMap use related Element's custom attributes values.

components/script/dom/webidls/DOMStringMap.webidl

@@ -7,6 +7,7 @@
 [OverrideBuiltins]
 interface DOMStringMap {
   getter DOMString (DOMString name);
+  [Throws]
   setter creator void (DOMString name, DOMString value);
   deleter void (DOMString name);
 };