Disable Picture-in-Picture steps should queue a task

[chrisn]

Raised by @marcoscaceres in #224, about the Disable Picture-in-Picture steps:

This should queue a task or something... promises can't be rejected synchronously here.

Consider, what should happen here:

const video = document.querySelector("video");
await video.requestPictureInPicture();
video.setAttribute("disablePictureInPicture", "");
video.removeAttribute("disablePictureInPicture");

That should probably have no effect. The task at the turn of the event loop should check if the attribute is still present and reject the promise(s) at that point.