This reverts commit c67b847fd43d97bcfe03ac455adbdb283b4872a1.

Bug 34315: avoid reading policies from /etc/firefox on Linux

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
  }
}

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.

Whenever a valid Onion-Location HTTP header (or corresponding HTML
<meta> http-equiv attribute) is found in a document load, we either
redirect to it (if the user opted-in via preference) or notify the
presence of an onionsite alternative with a badge in the urlbar.

We removed the addon in #32767, but it was still being loaded
from addonStartup.json.lz4 and throwing an error on startup
because its resource: location is not available anymore.

It's time for our rotation again: Move the backup key in the front
position and add a new backup key.