Make length property return useful values for DOM bindings functions

Source/core/xml/XMLHttpRequest.idl

 /*
  * Copyright (C) 2008, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 22 matching lines @@
     "document",
 //    FIXME: enable once support for json responseText is completed. (bug #73648)
 //    "json",
     "text"
 };
 
 [
     ActiveDOMObject,
     Constructor,
     CallWith=ScriptExecutionContext,
-    CustomConstructor,
+    CustomConstructor(optional XMLHttpRequestOptions options),
     EventTarget
 ] interface XMLHttpRequest {
     // From XMLHttpRequestEventTarget
     // event handler attributes
     attribute EventListener onabort;
     attribute EventListener onerror;
     attribute EventListener onload;
     attribute EventListener onloadend;
     attribute EventListener onloadstart;
     attribute EventListener onprogress;
@@ skipping 41 matching lines @@
 
     // EventTarget interface
     void addEventListener(DOMString type, 
                           EventListener listener, 
                           optional boolean useCapture);
     void removeEventListener(DOMString type, 
                              EventListener listener, 
                              optional boolean useCapture);
     [RaisesException] boolean dispatchEvent(Event evt);
 };