navigator.userAgent.indexOf('PhantomJS')是什么意思?
在vuejs的todoMVC的例子中看到的：

<!-- testing/benchmark only -->
    <script>
        var isPhantom = navigator.userAgent.indexOf('PhantomJS') > -1
        if (isPhantom) {
            localStorage.clear()
        } else {
            var now = window.performance && window.performance.now
                ? function () { return window.performance.now() }
                : Date.now
            var metrics = { beforeLoad: now() }
        }
    </script>
<!-- end testing/bench -->

demo文件地址：

https://github.com/vuejs/vue/blob/dev/examples/todomvc/index.html#L59