bufferStart(contentType) {
    if (!this.sourceBuffers_.size) {
      return null;
    }
    const ContentType = shaka.util.ManifestParserUtils.ContentType;
    if (contentType == ContentType.TEXT) {
      return this.textEngine_.bufferStart();
    }
    return shaka.media.TimeRangesUtils.bufferStart(
        this.getBuffered_(contentType));
  }
Gets the first timestamp in buffer for the given content type.

@param {shaka.util.ManifestParserUtils.ContentType} contentType
@return {?number} The timestamp in seconds, or null if nothing is buffered.