Introduce a flag to overwrite Date.now() with a fixed value

test/mjsunit/fixed-date.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.
+
+// Flags: --fixed-date=42940800
+
+var Jan1_2014_millis = 1388534400000;
+var fixed_date_secs = 42940800;
+assertEquals(Date.now(), Jan1_2014_millis + fixed_date_secs * 1000);
+assertEquals(Date.now(), Jan1_2014_millis + fixed_date_secs * 1000);
+assertEquals(new Date().valueOf(), Jan1_2014_millis + fixed_date_secs * 1000);