Add json responseType support to XMLHttpRequest

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 13 matching lines @@
  * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 enum XMLHttpRequestResponseType {
     "",
     "arraybuffer",
     "blob",
     "document",
-//    FIXME: enable once support for json responseText is completed. (bug #73648)
-//    "json",
+    "json",
     "text"
 };
 
 [
     GlobalContext=Window&WorkerGlobalScope,
     ActiveDOMObject,
     Constructor,
     ConstructorCallWith=ScriptExecutionContext,
     CustomConstructor(optional XMLHttpRequestOptions options)
 ] interface XMLHttpRequest : EventTarget {
@@ skipping 40 matching lines @@
 
     [SetterRaisesException] attribute XMLHttpRequestResponseType responseType;
     [CustomGetter, GetterRaisesException] readonly attribute Object response;
 
     [GetterRaisesException] readonly attribute unsigned short status;
     [GetterRaisesException] readonly attribute DOMString statusText;
 
     // Extension
     void overrideMimeType(DOMString override);
 };