updateIcon() {
    const Icons = shaka.ui.Enums.MaterialDesignIcons;
    if (this.isEnded() && this.video.duration) {
      this.button.textContent = Icons.REPLAY;
    } else {
      this.button.textContent = this.isPaused() ? Icons.PLAY : Icons.PAUSE;
    }
  }
Called when the button's icon needs to change.
To be overridden by subclasses.