Make `RegExp.prototype.flags` getter configurable

src/harmony-regexp.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 var $RegExp = global.RegExp;
 
 // -------------------------------------------------------------------
 
@@ skipping 10 matching lines @@
   if (this.multiline) result += 'm';
   if (this.unicode) result += 'u';
   if (this.sticky) result += 'y';
   return result;
 }
 
 function ExtendRegExpPrototype() {
   %CheckIsBootstrapping();
 
   %DefineAccessorPropertyUnchecked($RegExp.prototype, 'flags', RegExpGetFlags,
-                                   null, DONT_ENUM | DONT_DELETE);
+                                   null, DONT_ENUM);
   %SetNativeFlag(RegExpGetFlags);
 }
 
 ExtendRegExpPrototype();