In RFP mode, we do not support `PerformanceNavigationTiming`, so don't expose
it. In particular, `window.PerformanceNavigationTiming` should return
`undefined`.
Added a new method `PerformanceNavigationTiming::Enabled` which when used with
the WebIDL `Func` attribute allows us to toggle whether
`window.PerformanceNavigationTiming` is exposed.
Created
`dom/tests/mochitest/general/test_toggling_performance_navigation_timing.html`
to test whether the toggling works. Updated
`browser/components/resistfingerprinting/test/browser/browser_performanceAPI.js`
to create a new window each time `privacy.resistFingerprinting` is flipped so
this behavior does not leak into other tests.
Differential Revision: https://phabricator.services.mozilla.com/D73528