async getThumbnails(trackId, time) {
    const imageStream = await this.getBestImageStream_(trackId);
    if (!imageStream) {
      return null;
    }
    const referencePosition = imageStream.segmentIndex.find(time);
    if (referencePosition == null) {
      return null;
    }
    const reference = imageStream.segmentIndex.get(referencePosition);
    const dimensions = this.parseTilesLayout_(
        reference.getTilesLayout() || imageStream.tilesLayout);
    if (!dimensions) {
      return null;
    }
    return this.getThumbnailByReference_(reference, imageStream, time,
        dimensions);
  }
Return a Thumbnail object from a time.

If the player has not loaded content, this will return a null.

@param {?number} trackId
@param {number} time
@return {!Promise<?shaka.extern.Thumbnail>}
@export