Fix keyed element access wrt string wrappers

test/mjsunit/regress/regress-4296.js

+// Copyright 2015 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.
+
+(function () {
+  var o = new String("ab");
+  function store(o, i, v) { o[i] = v; }
+  function load(o, i) { return o[i]; }
+
+  // Initialize the IC.
+  store(o, 2, 10);
+  load(o, 2);
+
+  store(o, 0, 100);
+  assertEquals("a", load(o, 0));
+})();
+
+(function () {
+  var o = {__proto__: new String("ab")};
+  function store(o, i, v) { o[i] = v; }
+  function load(o, i) { return o[i]; }
+
+  // Initialize the IC.
+  store(o, 2, 10);
+  load(o, 2);
+
+  store(o, 0, 100);
+  assertEquals("a", load(o, 0));
+})();
+
+(function () {
+  "use strict";
+  var o = {__proto__: {}};
+  function store(o, i, v) { o[i] = v; }
+
+  // Initialize the IC.
+  store(o, 0, 100);
+  o.__proto__.__proto__ = new String("bla");
+  assertThrows(function () { store(o, 1, 100) });
+})();