getContentTypeState_(contentType) {
    let contentTypeState = this.contentTypeStates_.get(contentType);
    if (!contentTypeState) {
      contentTypeState = {
        qualityChangePositions: [],
        currentQuality: null,
        contentType: contentType,
      };
      this.contentTypeStates_.set(contentType, contentTypeState);
    }
    return contentTypeState;
  }
Get the ContentTypeState for a contentType, creating a new
one if necessary.

@param {!string} contentType
 The contend type e.g. "video" or "audio".
@return {!shaka.media.QualityObserver.ContentTypeState}
@private