The measure tool is moving to PageSpeed Insights
If you've been to this page before, you probably expected to see a field where you can input a URL and get performance diagnostics for that page. This tool has been removed because of the significant overlap it has with PageSpeed Insights.
Going forward, PageSpeed Insights will be what you need. It will give you detailed lab insights into your website's performance, but also field data insights, which is important as it allows you to assess your website's user experience from the perspective of your website's actual users.