postIdTokenToSessionLogin = function(url, idToken, csrfToken) {
  // POST to session login endpoint.
  return $.ajax({
    type:'POST',
    url: url,
    data: {idToken: idToken, csrfToken: csrfToken},
    contentType: 'application/x-www-form-urlencoded'
  });
}
@param {string} url The session login endpoint.
@param {string} idToken The ID token to post to backend.
@param {?string} csrfToken The CSRF token to send to backend.
@return {any} A jQuery promise that resolves on completion.