WPT navigation-timing/supported_navigation_type.window.js never sees a navigation event

[pshaughn]

This test has a preamble that uses supportedEntryTypes for feature detection; past that preamble, it creates a 'navigation' PerformanceObserver with the resolution of the test inside the observer's callback. I don't know exactly what navigation event is supposed to fire that callback, but Chrome and Firefox pass this test; Servo, if running the test at all, never gets the expected event and times out.