User Timings broken in Chrome and other browsers

More info at window.performance.mark not show for several browsers · Issue #446 · WPO-Foundation/webpagetest · GitHub

tl;dr: As of Chrome 43 (released on May 15th), JSON.stringify() will no longer work on window.performance.marks. And that makes it not show any user timings in the test result.

Should be fixed as of earlier today.