chromium/chrome/test/data/media/picture-in-picture/autopip-toggle-registration.html

<!DOCTYPE html>
<head>
<meta charset="utf-8">
<title>Autopip Register Picture-in-Picture action</title>
</head>
<body>
</body>
<script>
function register() {
  navigator.mediaSession.setActionHandler('enterpictureinpicture', _=>{});
}

function unregister() {
  navigator.mediaSession.setActionHandler('enterpictureinpicture', null);
}
</script>