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.

Restrict the Enterprise Policies mechanism to only consult a
policies.json file (avoiding the Windows Registry and macOS's
file system attributes).

Add a few disabledByPolicy() checks to the update service to
avoid extraneous (and potentially confusing) log messages when
updates are disabled by policy.

Sample content for distribution/policies.json:
{
  "policies": {
    "DisableAppUpdate": true
  }
}

This reverts commit d362cff17d44a82534ea553cbf50fa76906cc218.

A custom HTTPS Everywhere update channel is installed,
which provides rules for locally redirecting some memorable
.tor.onion URLs to non-memorable .onion URLs.

When these redirects occur, we also rewrite the URL in the urlbar
to display the human-memorable hostname instead of the actual
.onion.