get downloadProgress() {
        let percent;
        if (this[kResponseSize]) {
            percent = this[kDownloadedSize] / this[kResponseSize];
        }
        else if (this[kResponseSize] === this[kDownloadedSize]) {
            percent = 1;
        }
        else {
            percent = 0;
        }
        return {
            percent,
            transferred: this[kDownloadedSize],
            total: this[kResponseSize]
        };
    }
Progress event for downloading (receiving a response).