Revert "Use new syntax for DOMException. (#780)"

This reverts commit 21ffd9b.

Author: bzbarsky

index.bs

@@ -14146,10 +14146,9 @@ fragment:
 
 <pre class="idl">
 [Exposed=(Window,Worker),
+ Constructor(optional DOMString message = "", optional DOMString name = "Error"),
  Serializable]
 interface DOMException { // but see below note about ECMAScript binding
-  constructor(optional DOMString message = "", optional DOMString name = "Error");
-
   readonly attribute DOMString name;
   readonly attribute DOMString message;
   readonly attribute unsigned short code;