addSeekBar_() {
    if (this.config_.addSeekBar) {
      this.seekBar_ = shaka.ui.ControlsPanel.seekBarFactory_.create(
          this.bottomControls_, this);
      this.elements_.push(this.seekBar_);
    } else {
      // Settings menus need to be positioned lower if the seekbar is absent.
      for (const menu of this.menus_) {
        menu.classList.add('shaka-low-position');
      }
      // Tooltips need to be positioned lower if the seekbar is absent.
      const controlsButtonPanel = this.controlsButtonPanel_;
      if (controlsButtonPanel.classList.contains('shaka-tooltips-on')) {
        controlsButtonPanel.classList.add('shaka-tooltips-low-position');
      }
    }
  }
Adds a seekbar depending on the configuration.
By default an instance of shaka.ui.SeekBar is created
This behaviour can be overridden by providing a SeekBar factory using the
registerSeekBarFactory function.

@private