open(estimate) {
    this.estimatedTotal_ += estimate;

    const id = this.nextId_;
    this.nextId_++;

    this.pending_.set(id, estimate);
    return id;
  }
Open a new download in the progress estimator. This will create an entry so
that we can track the download progress.

This will return an id for the download. This id must be passed to |close|
in order for the |close| to be paired with this call to |open|.

@param {number} estimate
@return {number}