IDL: Support 'stringifier' keyword

Source/core/css/WebKitCSSMatrix.idl

 /*
  * Copyright (C) 2008, 2010 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 82 matching lines @@
                                                 [Default=Undefined] optional double angle);
 
     // Returns this matrix skewed along the X axis by the passed values.
     // Passing a NaN will use a value of 0.
     [Immutable] WebKitCSSMatrix skewX([Default=Undefined] optional double angle);
 
     // Returns this matrix skewed along the Y axis by the passed values.
     // Passing a NaN will use a value of 0.
     [Immutable] WebKitCSSMatrix skewY([Default=Undefined] optional double angle);
 
-    [NotEnumerable] DOMString toString();
+    [NotEnumerable] stringifier;
 };