Function getSentinel_

src/iframe-helper.js:398–400

* Gets the sentinel string. * @param {!Element} iframe The iframe. * @param {boolean=} opt_is3P set to true if the iframe is 3p. * @return {string} Sentinel string. * @private

(iframe, opt_is3P)

 * @private
 */
function getSentinel_(iframe, opt_is3P) {
  return opt_is3P ? iframe.getAttribute('data-amp-3p-sentinel') : 'amp';
}

/**
 * JSON parses event.data if it needs to be