An older version of the spec expects performance.timing to look like this: https://www.w3.org/TR/navigation-timing/#sec-navigation-timing-interface
The current spec looks like this: https://www.w3.org/TR/navigation-timing-2/#sec-PerformanceNavigationTiming
Some WPT tests, like navigation-timing/test_timing_attributes_order.html, still want the fields of the obsolete version to be populated; Servo is doing just the fields of the new version and thus fails those tests.