constructor(uniqueId) {
    /**
     * @private @const
     * {!Map<number, AbstractOverlayStats.LayerStatsForZoomLevel>}
     * Statistics for tiles at each zoom level.
     */
    this.statsByZoom_ = new Map();
    this.uniqueId_ = uniqueId;
  }
@param{string} uniqueId An id unique to this map overlay.