async acquire(identifier) {
    shaka.log.v2(identifier + ' has requested mutex');
    if (this.acquiredIdentifier) {
      await new Promise((resolve) => this.unlockQueue.push(resolve));
    }
    this.acquiredIdentifier = identifier;
    shaka.log.v2(identifier + ' has acquired mutex');
  }
Acquires the mutex, as soon as possible.
@param {string} identifier
@return {!Promise}