getStatsForZoom_(zoom) {
    if (!this.statsByZoom_.has(zoom)){
      this.statsByZoom_.set(zoom, {
        throttleCount: 0,
        errorCount: 0,
        tileLatencies: [],
      });
    }
    return this.statsByZoom_.get(zoom);
  }
Layer stats for tiles at a given zoom.
@param {number} zoom The zoom level.
@return {AbstractOverlayStats.LayerStatsForZoomLevel}
@private