Make sure EME is no tracking risk in Tor Browser

The EME architecture got uplifted to Firefox 37 (https://bugzilla.mozilla.org/show_bug.cgi?id=1137045) and is included in ESR 38 as well. We should make sure there are no accompanying tracking/fingerprinting risks. The best plan is probably to disable EME as Mozilla is doing in its ESR 38 release. We may need a custom patch as Mozilla is basically enabling it

#if !defined(MOZ_UPDATE_CHANNEL) || MOZ_UPDATE_CHANNEL != esr

While we may want to take a deeper look at it when we switch to ESR 45 we should make sure that everything related to EME is really disabled if the respective prefs are set to false.