getEstimatedProgress() {
    return this.estimatedTotal_ == 0 ?
           0 :
           this.estimatedDownloaded_ / this.estimatedTotal_;
  }
Get the current estimate for how much progress we've made downloading the
content. Progress will be between 0 and 1.

Depending on the order of calls to |open| and |close|,
|getEstimatedProgress| will fluctuate and is not guaranteed to always be
increasing.

@return {number}