irst, then fall back to the basic config.
  var configs = [offlineConfig, basicConfig];

  var support = {};
  testKeySystems.forEach(function(keySystem) {
    var p = navigator.requestMediaKeySystemAccess(keySystem, configs)
        .then(function(access) {
          // We can't (yet) trust every browser to report the session types they
          // support in access.getConfiguration().  Therefore, we create a
          // MediaKeys object and try to create an offline session.
          // https://goo.gl/g