Expose Array.prototype.values behind a flag and stage it

test/mjsunit/es6/array-prototype-values.js

+// 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.
+//
+// Flags: --harmony-array-prototype-values
+
+// Functionality of the values iterator is tested elsewhere; this test
+// merely verifies that the 'values' property is set up correctly.
+var valuesDesc = Object.getOwnPropertyDescriptor(Array.prototype, 'values');
+assertEquals('object', typeof valuesDesc);
+assertSame(Array.prototype[Symbol.iterator], valuesDesc.value);
+assertTrue(valuesDesc.configurable);
+assertTrue(valuesDesc.writable);
+assertFalse(valuesDesc.enumerable);
+assertTrue(Array.prototype[Symbol.unscopables].values);