function signInSuccessCallback(redirect, opt_manualRedirect) {
  return function(user, credential, redirectUrl) {
    // Save sign in callback parameters to confirm callback invoked.
    signInCallbackUser = user;
    signInCallbackRedirectUrl = redirectUrl;
    signInCallbackCredential = credential;
    // Developer manually redirects and redirect URL available.
    if (opt_manualRedirect && redirectUrl) {
      firebaseui.auth.util.goTo(redirectUrl);
    }
    return redirect;
  };
}
@param {boolean} redirect The return status of the success callback.
@param {boolean=} opt_manualRedirect Whether the developer manually
    redirects to redirectUrl.
@return {!firebaseui.auth.widget.Config.signInSuccessCallback} sign in
    success callback function.