Function postMessage

src/iframe-helper.js:353–361

(iframe, type, object, targetOrigin, opt_is3P)

 * @param {boolean=} opt_is3P set to true if the iframe is 3p.
 */
export function postMessage(iframe, type, object, targetOrigin, opt_is3P) {
  postMessageToWindows(
    iframe,
    [{win: iframe.contentWindow, origin: targetOrigin}],
    type,
    object,
    opt_is3P
  );
}

/**
 * Posts an identical message to multiple target windows with the same