[runtime] Make sure %ForInPrepare returns the correct length.

test/mjsunit/regress/regress-3650-2.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
+
+var a = {}
+var b = {}
+a.x = 1;
+a.y = 1;
+b.x = 1;
+
+function foo(c) {
+  var s = 0;
+  for (var p in c) { s++; }
+  return s;
+}
+
+assertEquals(2, foo(a));
+assertEquals(1, foo(b));
+%OptimizeFunctionOnNextCall(foo);
+assertEquals(2, foo(a));
+assertEquals(1, foo(b));