servo/tests/wpt/web-platform-tests/feature-policy/experimental-features/resources/common.js

const url_base = "/feature-policy/experimental-features/resources/";
window.messageResponseCallback = null;

function setFeatureState(iframe, feature, origins) {
    iframe.setAttribute("allow", `${feature} ${origins};`);
}

// Returns a promise which is resolved when the <iframe> is navigated to |url|
// and "load" handler has been called.
function loadUrlInIframe(iframe, url) {
  return new Promise((resolve) => {
    iframe.addEventListener("load", resolve);
    iframe.src = url;
  });
}

// Posts |message| to |target| and resolves the promise with the response coming
// back from |target|.
function sendMessageAndGetResponse(target, message) {
  return new Promise((resolve) => {
    window.messageResponseCallback = resolve;
    target.postMessage(message, "*");
  });
}


function onMessage(e) {
  if (window.messageResponseCallback) {
    window.messageResponseCallback(e.data);
    window.messageResponseCallback = null;
  }
}

window.addEventListener("message", onMessage);