async clear(contentType) {
    const ContentType = shaka.util.ManifestParserUtils.ContentType;
    if (contentType == ContentType.TEXT) {
      if (!this.textEngine_) {
        return;
      }
      await this.textEngine_.remove(0, Infinity);
    } else {
      // Note that not all platforms allow clearing to Infinity.
      await this.enqueueOperation_(
          contentType,
          () => this.remove_(contentType, 0, this.mediaSource_.duration),
          null);
      if (this.needSplitMuxedContent_) {
        await this.enqueueOperation_(
            ContentType.AUDIO,
            () => this.remove_(
                ContentType.AUDIO, 0, this.mediaSource_.duration),
            null);
      }
    }
  }
Enqueue an operation to clear the SourceBuffer.

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