code: function onPopupRedirectProviderClick(event) {
    var providerId = $(event.currentTarget).data('provider');
    var provider = null;
    switch (providerId) {
      case 'google.com':
        provider = new firebase.auth.GoogleAuthProvider();
        break;
      case 'facebook.com':
        provider = new firebase.auth.FacebookAuthProvider();
        break;
      case 'github.com':
        provider = new firebase.auth.GithubAuthProvider();
        break;
      case 'twitter.com':
        provider = new firebase.auth.TwitterAuthProvider();
        break;
      default:
        return;
    }
    signInWithPopupRedirect(provider);
  }
docs: Performs the corresponding popup/redirect action based on user's selection.
  @param {!jQuery.Event} event The jQuery event object.