checkForManifest(aWindow) {
  // Only top-level browsing contexts are valid.
  if (!aWindow || aWindow.top !== aWindow) {
    return false;
  }
  const elem = aWindow.document.querySelector("link[rel~='manifest']");
  // Only if we have an element and a non-empty href attribute.
  if (!elem || !elem.getAttribute("href")) {
    return false;
  }
  return true;
}
PK