amp-apester-media: Element mutation outside mutation phase

amp-apester-media inserts its iframe inside #layoutCallback, but because it’s inside a Promise’s then chain, it’s outside of a mutation phase.

layoutCallback() {
  this.element.classList.add('amp-apester-container');
  // This is inside the mutation phase

  return this.queryMedia_().then(response => {
    // We're no longer inside mutation phase, due to promise's
    // async resolution.
    this.element.appendChild(iframe);
  });
}

[aghassemi]

fixed by #13741

[OmriKeret]

Hi guys, I will gladly fix it with our next PR (ETA - end of next week).

@aghassemi Is it soon enough?