Introduce a monotonic_clock module for Mojo.js.

mojo/apps/js/bindings/monotonic_clock_unittests.js

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+define([
+  "console",
+  "gin/test/expect",
+  "monotonic_clock",
+  "timer",
+  "mojo/apps/js/bindings/threading"
+], function(console, expect, monotonicClock, timer, threading) {
+  var global = this;
+  var then = monotonicClock.seconds();
+  var t = timer.createOneShot(100, function() {
+    var now = monotonicClock.seconds();
+    expect(now).toBeGreaterThan(then);
+    global.result = "PASS";
+    threading.quit();
+  });
+});