FileReader.result attribute should use IDL union types to avoid using custom binding.

Source/core/fileapi/FileReader.idl

 /*
  * Copyright (C) 2010 Google Inc.  All rights reserved.
  * Copyright (C) 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 34 matching lines @@
 
     // async read methods
     [RaisesException] void readAsArrayBuffer(Blob blob);
     [RaisesException] void readAsBinaryString(Blob blob);
     [RaisesException] void readAsText(Blob blob, optional DOMString encoding);
     [RaisesException] void readAsDataURL(Blob blob);
 
     void abort();
 
     // file data
-    [Custom=Getter] readonly attribute object result;
+    readonly attribute (DOMString or ArrayBuffer)? result;
 
     readonly attribute FileError error;
 
     attribute EventHandler onloadstart;
     attribute EventHandler onprogress;
     attribute EventHandler onload;
     attribute EventHandler onabort;
     attribute EventHandler onerror;
     attribute EventHandler onloadend;
 };