Add Chrome mediakey support

[alecgorge]

It looks like supporting this Chrome extension wouldn't be too bad:

// Web Page Media Control API interaction (v0.4)
// https://github.com/feedbee/web-page-media-control-api-spec
// for Key Socket Media Keys Chrome extension
// https://chrome.google.com/webstore/detail/key-socket-media-keys/fphfgdknbpakeedbaenojjdcdoajihik?hl=en

document.addEventListener("MediaPlayPause", function () {
  playerCollection.send("toggle");
});
document.addEventListener("MediaStop", function () {
  playerCollection.send("stop");
});
document.addEventListener("MediaPrev", function () {
  var player = playerCollection.getPrevPlayer();
  if (player) {
      player.send("play");
  }
});
document.addEventListener("MediaNext", function () {
  var player = playerCollection.getNextPlayer();
  if (player) {
      player.send("play");
  }
});

This could be part of a bigger effort to support the Android Chrome stuff as well I suppose