onButtonClicked_(event) {
    event.preventDefault();
    if (event.target === this.next) {
      this.ws_.goNext();
    } else if (event.target === this.prev) {
      this.ws_.goPrev();
    } else {
      this.ws_.toggleZoom();
    }
  }
Handles clicks on the next/prev buttons.
@param {MouseEvent} event
@private