addFastForwardButtonOnControlsContainer_() {
    const hiddenFastForwardContainer = shaka.util.Dom.createHTMLElement('div');
    hiddenFastForwardContainer.classList.add(
        'shaka-hidden-fast-forward-container');
    this.controlsContainer_.appendChild(hiddenFastForwardContainer);

    /** @private {shaka.ui.HiddenFastForwardButton} */
    this.hiddenFastForwardButton_ =
        new shaka.ui.HiddenFastForwardButton(hiddenFastForwardContainer, this);
    this.elements_.push(this.hiddenFastForwardButton_);
  }
Add fast-forward button on Controls container for moving video some
seconds ahead when the video is tapped more than once, video seeks ahead
some seconds for every extra tap.
@private