Add support for [TreatNullAs] extended attribute for reflected attributes

Source/bindings/tests/idls/TestObject.idl

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary formstrArg, 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.
@@ skipping 45 matching lines @@
     attribute unsigned long long       unsignedLongLongAttr;
     attribute DOMString                stringAttr;
     attribute EventHandler             eventHandlerAttr;
     [MeasureAs=TestFeature] attribute TestObject testObjAttr;
 
     // WK_ucfirst, WK_lcfirst exceptional cases.
     attribute TestObject                  XMLObjAttr;
     attribute boolean                  create;
 
     // Reflected DOM attributes
-    [Reflect] attribute DOMString reflectedStringAttr;
+    [Reflect, TreatNullAs=NullString] attribute DOMString reflectedStringAttr;
     [Reflect] attribute long reflectedIntegralAttr;
     [Reflect] attribute unsigned long reflectedUnsignedIntegralAttr;
     [Reflect] attribute boolean reflectedBooleanAttr;
-    [Reflect, URL] attribute DOMString reflectedURLAttr;
-    [Reflect=customContentStringAttr] attribute DOMString reflectedStringAttr;
+    [Reflect, TreatNullAs=NullString, URL] attribute DOMString reflectedURLAttr;
+    [Reflect=customContentStringAttr, TreatNullAs=NullString] attribute DOMString reflectedStringAttr;
     [Reflect=customContentIntegralAttr] attribute long reflectedCustomIntegralAttr;
     [Reflect=customContentBooleanAttr] attribute boolean reflectedCustomBooleanAttr;
-    [Reflect=customContentURLAttr, URL] attribute DOMString reflectedCustomURLAttr;
+    [Reflect=customContentURLAttr, TreatNullAs=NullString, URL] attribute DOMString reflectedCustomURLAttr;
 
     // TypedArray attribute
     attribute Float32Array typedArrayAttr;
 
     // Methods
     void    voidMethod();
     void    voidMethodWithArgs(long longArg, DOMString strArg, TestObject objArg);
     long    longMethod();
     long    longMethodWithArgs(long longArg, DOMString strArg, TestObject objArg);
     [MeasureAs=TestFeature] TestObject objMethod();
@@ skipping 214 matching lines @@
 
     [DeprecateAs=Method] void deprecatedMethod();
     [DeprecateAs=StaticMethod] static void deprecatedStaticMethod();
 
     [DeprecateAs=Constant] const unsigned short DEPRECATED_CONSTANT = 1;
 };
 
 // The following comment should not generate any code
 // TestObject implements TestImplements;