onKeyDown(event) {
    if (!this.isZoomed_ && Keys.MINUS.some(key => key === event.which)) {
      this.zoomIn();
    } else if (this.isZoomed_ &&
      (Keys.PLUS.some(key => key === event.which) ||
        event.which === Keys.ESCAPE)) {
      this.zoomOut();
    }
  }
On key down handler. Will decide if Zoom in or out
@param {Event} event Key down event.