getUniqueTileId_(coord, z) {
    var tileId = [coord.x, coord.y, z, this.tileCounter++].join('-');
    var sourceId = this.tileSource.getUniqueId();
    return [tileId, sourceId].join('-');
  }
Generates a unique for this tile from the tile source. It includes:

1) A unique counter string to the tileid to make sure that
   repeated requests for the same tile and cancellations thereof
   (which may occur if the user quickly moves the map around)
   don't overwrite each other's state.
2) A unique token for this overlay's source to differentiate its tile
   requests from other tile requests for other layers with the same map ID.

@param {!google.maps.Point} coord The position of the tile.
@param {number} z The zoom level of the tile.
@return {string} A unique tile ID.
@private