setAppendWindow_(contentType, appendWindowStart, appendWindowEnd) {
    const sourceBuffer = this.sourceBuffers_.get(contentType);
    if (sourceBuffer.appendWindowEnd !== appendWindowEnd ||
        sourceBuffer.appendWindowStart !== appendWindowStart) {
      // You can't set start > end, so first set start to 0, then set the new
      // end, then set the new start.  That way, there are no intermediate
      // states which are invalid.
      sourceBuffer.appendWindowStart = 0;
      sourceBuffer.appendWindowEnd = appendWindowEnd;
      sourceBuffer.appendWindowStart = appendWindowStart;
    }

    // Fake an 'updateend' event to resolve the operation.
    this.onUpdateEnd_(contentType);
  }
Set the SourceBuffer's append window end.
@param {shaka.util.ManifestParserUtils.ContentType} contentType
@param {number} appendWindowStart
@param {number} appendWindowEnd
@private