bufferedAheadOf(t) {
    if (this.bufferEnd_ == null || this.bufferEnd_ < t) {
      return 0;
    }

    goog.asserts.assert(
        this.bufferStart_ != null,
        'start should not be null if end is not null');

    return this.bufferEnd_ - Math.max(t, this.bufferStart_);
  }
@param {number} t A timestamp
@return {number} Number of seconds ahead of 't' we have buffered