getSummaryList() {
    let summaryList = [];
    this.statsByZoom_.forEach((stats, zoom) => summaryList.push({
      layerId: this.uniqueId_,
      zoomLevel: zoom,
      tileLatencies: stats.tileLatencies,
      throttleCount: stats.throttleCount,
      errorCount: stats.errorCount,
    }));
    return summaryList;
  }
Returns a list for stats for this overlay. Each entry in the list contains
tile data for tiles of the same zoom level.
@return {!Array<AbstractOverlayStats.Summary>} current stats