[stubs] Fix AccessorInfo mixup in KeyedStoreGeneric

test/mjsunit/regress/regress-crbug-668101.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: --allow-natives-syntax
+
+function f(a, i, v) {
+  a[i] = v;
+}
+
+f("make it generic", 0, 0);
+
+var a = new Array(3);
+// Fast properties.
+f(a, "length", 2);
+assertEquals(2, a.length);
+
+// Dictionary properties.
+%OptimizeObjectForAddingMultipleProperties(a, 1);
+f(a, "length", 1);
+assertEquals(1, a.length);