Bug 215161

Reproduce:
1. open https://mantou132.github.io/javascript-learn/stream.html
2. click pip button
3. Focus on other apps and let Safari enter the background

expect: The video plays smoothly in the pip window

actual: video paused

```
  const ctx = canvas.getContext('2d');
  const tick = () => {
    ctx.canvas.width = ctx.canvas.width;
    ctx.fillStyle = 'white';
    ctx.fillRect(0, 0, ctx.canvas.width, ctx.canvas.height);
    ctx.fillStyle = 'black';
    ctx.font = '50px serif';
    ctx.fillText(Math.floor(performance.now()), 0, 50);
    requestAnimationFrame(tick);
  }
  tick();
  const stream = canvas.captureStream();
  video.srcObject = stream;
  video.play();
  btn.onclick = () => video.requestPictureInPicture();
```