async flush(contentType) {
    // Flush the pipeline.  Necessary on Chromecast, even though we have removed
    // everything.
    const ContentType = shaka.util.ManifestParserUtils.ContentType;
    if (contentType == ContentType.TEXT) {
      // Nothing to flush for text.
      return;
    }
    await this.enqueueOperation_(
        contentType,
        () => this.flush_(contentType),
        null);
    if (this.needSplitMuxedContent_) {
      await this.enqueueOperation_(
          ContentType.AUDIO,
          () => this.flush_(ContentType.AUDIO),
          null);
    }
  }
Enqueue an operation to flush the SourceBuffer.
This is a workaround for what we believe is a Chromecast bug.

@param {shaka.util.ManifestParserUtils.ContentType} contentType
@return {!Promise}