ld be applied to
// nested content.

callbacks = {};

window.addEventListener('message', ev => {
  var id = ev.data.id;
  if (id && callbacks[id]) {
    callbacks[id](ev.data.requiredPolicy || null);
  }
});

async_test(t => {
  var iframe = document.createElement('iframe');
  iframe.src = "/document-policy/echo-policy.py?id=1";
  callbacks["1"] = t.step_func_done(result => {
    assert_equals(result, "lossless-images-max-bpp=1.0");
  });
  document.body.appendChild(iframe);
}, "Child frame with no explicit