More preferences ID cleanups
Merge remote-tracking branch 'upstream/master' into tom/prefs-id-cleanup-2
Improved getClientID and getPreferenceStorage
Fixes after review
Using getExtensionPaths and updated handleClientIdChange
Removing PreferencesManager-test.js as a headless test
Temporarily removing the tests to fix the Travis build
Merge pull request #3101 from TomMalbran/tom/prefs-id-cleanup-2

Preferences ID cleanups - Part II