[json] Allow any callable object for toJSON.

test/mjsunit/regress/regress-crbug-595738.js

Index: test/mjsunit/regress/regress-crbug-595738.js
diff --git a/test/message/arrow-invalid-rest.js b/test/mjsunit/regress/regress-crbug-595738.js
similarity index 63%
copy from test/message/arrow-invalid-rest.js
copy to test/mjsunit/regress/regress-crbug-595738.js
index 870dbe9f542ea95050514227feaf9b2299ca516d..d2409ab4aea6a7077cf2720c3b24c37742e85ab4 100644
--- a/test/message/arrow-invalid-rest.js
+++ b/test/mjsunit/regress/regress-crbug-595738.js
@@ -1,8 +1,7 @@
 // Copyright 2016 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.
-//
-//
 
-var f = (...x = 10) => x;
-f(1, 2, 3, 4, 5);
+function foo() { return 1; }
+var x = {toJSON: foo.bind()};
+assertEquals("1", JSON.stringify(x));