Make [TreatNullAs=NullString] DOMString arguments nullable instead

Source/core/storage/StorageEvent.idl

 /*
  * Copyright (C) 2008, 2009 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 18 matching lines @@
     [InitializedByEventConstructor] readonly attribute DOMString key;
     [InitializedByEventConstructor, TreatReturnedNullStringAs=Null] readonly attribute DOMString oldValue;
     [InitializedByEventConstructor, TreatReturnedNullStringAs=Null] readonly attribute DOMString newValue;
     [InitializedByEventConstructor] readonly attribute DOMString url;
     [InitializedByEventConstructor] readonly attribute Storage? storageArea;
 
     void initStorageEvent([Default=Undefined] optional DOMString typeArg,
                           [Default=Undefined] optional boolean canBubbleArg,
                           [Default=Undefined] optional boolean cancelableArg,
                           [Default=Undefined] optional DOMString keyArg,
-                          [Default=Undefined, TreatNullAs=NullString] optional DOMString oldValueArg,
-                          [Default=Undefined, TreatNullAs=NullString] optional DOMString newValueArg,
+                          [Default=Undefined] optional DOMString? oldValueArg,
+                          [Default=Undefined] optional DOMString? newValueArg,
                           [Default=Undefined] optional DOMString urlArg,
                           [Default=Undefined] optional Storage storageAreaArg);
 
     // Needed once we support init<blank>EventNS
     // void initStorageEventNS(DOMString namespaceURI, DOMString typeArg, boolean canBubbleArg, boolean cancelableArg, DOMString keyArg, DOMString oldValueArg, DOMString newValueArg, DOMString urlArg, Storage storageAreaArg);
 };