(ESR68) Bug 1511941 - Don't expose PerformanceNavigationTiming in RFP mode

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.