addTileCallback(callback) {
    return /** @type {!Object} */ (goog.events.listen(
        this, ee.layers.AbstractOverlay.EventType.TILE_LOAD, callback));
  }
Adds a callback to be fired each time a tile is loaded.
@param {function(ee.layers.TileLoadEvent)} callback The function to call.
@return {!Object} An ID which can be passed to removeTileCallback().
@export