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;
}
Blob upload/download speed summary.
Trigger 'progress' event every progress updates.