popFromQueue_(contentType) {
    goog.asserts.assert(this.queues_.has(contentType), 'Queue should exist');
    // Remove the in-progress operation, which is now complete.
    this.queues_.get(contentType).shift();
    this.startOperation_(contentType);
  }
Pop from the front of the queue and start a new operation.
@param {shaka.util.ManifestParserUtils.ContentType} contentType
@private