code: function SpeedSummary (name) {
    this.name = name;
    this._startTime = Date.now();
    this._timeWindowInSeconds = 10;
    this._timeWindow = this._timeWindowInSeconds * 1000;
    this._totalWindowSize = 0;
    this._speedTracks = new Array(this._timeWindowInSeconds);
    this._speedTrackPtr = 0;
    this.totalSize = undefined;
    this.completeSize = 0;
  }
docs: Blob upload/download speed summary.
  Trigger 'progress' event every progress updates.