[web-animations-2][css-animations-2] Allow controlling video playback via an animation timeline

[flackr]

Similar to #9110, authors often want the ability to drive videos by animation timelines. See #11587 and #6861 (comment).

We'll need to figure out the details, but this could be something like

document.querySelector('#my-video').timeline = new ViewTimeline(...);

Possibly also with css support, e.g.:

#my-video {
  video-timeline: view();
}