Fix: KeyedStoreGeneric must check for writable array length

test/mjsunit/regress/regress-5669.js

+// Copyright 2017 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 f(a, i, v) { a[i] = v; }
+f("make it generic", 0, 0);
+
+var a = new Array();
+Object.defineProperty(a, "length", {value: 3, writable: false});
+print(JSON.stringify(Object.getOwnPropertyDescriptor(a, "length")));
+assertEquals(3, a.length);
+f(a, 3, 3);
+assertFalse(Object.getOwnPropertyDescriptor(a, "length").writable);
+assertEquals(3, a.length);
+
+var b = new Array();
+b.length = 3;
+Object.freeze(b);
+assertEquals(3, b.length);
+f(b, 3, 3);
+assertEquals(3, b.length);