setCompletionPercent(percent) {
    if (isNaN(this.completionPercent_)) {
      this.completionPercent_ = percent;
    } else {
      this.completionPercent_ = Math.max(this.completionPercent_, percent);
    }
  }
Record the current completion percent. This is the "high water mark", so it
will store the highest provided completion percent.

@param {number} percent