Bound functions should also have configurable length

test/mjsunit/regress/regress-1419.js

 // Copyright 2011 the V8 project authors. 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.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 26 matching lines @@
 assertEquals(1, f1.length);
 
 // the object we bind to can be any object
 f2.bind(foo);
 
 assertEquals(1, f1.length);
 
 var desc = Object.getOwnPropertyDescriptor(f1, 'length');
 assertEquals(false, desc.writable);
 assertEquals(false, desc.enumerable);
-assertEquals(false, desc.configurable);
+assertEquals(true, desc.configurable);
+
+Object.defineProperty(f1, 'length', {
+  value: 'abc',
+  writable: true
+});
+assertEquals('abc', f1.length);
+f1.length = 42;
+assertEquals(42, f1.length);