async onTracksChanged_() {
    if (!this.controls.isPiPAllowed()) {
      shaka.ui.Utils.setDisplay(this.pipButton_, false);
      if (this.controls.isPiPEnabled()) {
        await this.controls.togglePiP();
      }
    } else if (this.player && this.player.isAudioOnly()) {
      shaka.ui.Utils.setDisplay(this.pipButton_, false);
      if (this.controls.isPiPEnabled()) {
        await this.controls.togglePiP();
      }
    } else {
      shaka.ui.Utils.setDisplay(this.pipButton_, true);
    }
  }
Display the picture-in-picture button only when the content contains video.
If it's displaying in picture-in-picture mode, and an audio only content is
loaded, exit the picture-in-picture display.
@return {!Promise}
@private