ff-addon-script: Use atob and btoa in privelaged scope (no access to window)

_privelaged-atob-btoa.js

const {utils: Cu} = Components;
const { atob, btoa } = Cu.import("resource://gre/modules/Services.jsm");
//can now use atob and btoa freely

//if it is utf-8 then do the following:

function atobUTF8(data) {
    return decodeURIComponent(escape(atob(data)))
}

function btoaUTF8(data) {
    return btoa(unescape(encodeURIComponent(data)))
}