changeType(contentType, mimeType, transmuxer) {
    return this.enqueueOperation_(
        contentType,
        () => this.change_(contentType, mimeType, transmuxer),
        null);
  }
Enqueue an operation to prepare the SourceBuffer to parse a potentially new
type or codec.

@param {shaka.util.ManifestParserUtils.ContentType} contentType
@param {string} mimeType
@param {?shaka.extern.Transmuxer} transmuxer
@return {!Promise}