constructor(hidden, visibilityState) {
    super(EventType.VISIBILITYCHANGE);

    /**
     * Whether the page is hidden.
     * @type {boolean}
     */
    this.hidden = hidden;

    /**
     * A more detailed visibility state.
     * @type {!PageVisibilityState}
     */
    this.visibilityState = visibilityState;
  }
Constructs a new PageVisibilityEvent.
@param {boolean} hidden Whether the page is hidden.
@param {!PageVisibilityState} visibilityState A more detailed visibility
    state.