- Electron version: 1.4.3
- Operating system:  @win7
- description:

working on a download app with electron and angular. 
By using webContents.downloadUrl, webContents.session.on('will-download', handler) I can make download function work well, however, I could not find a api to save download progress before closing electron app so that I can restore download when this app is started next time.

Excepted Result:
1. user click 'download' button to download a 1G file.
2. after 500M is downloaded, user quit the app
3. one hour later, user open this app again, and downloading can automatically be restored

atom/browser/api/atom_api_download_item.cc
return download_item_->GetTotalBytes();
}
std::string DownloadItem::GetMimeType() const {
  return download_item_->GetMimeType();
}
bool DownloadItem::HasUserGesture() const {
  return download_item_->HasUserGesture();
}
std::string DownloadItem::GetFilename() const {
  return base::UTF16ToUTF8(net::GenerateFileName(GetURL(),
                           GetContentDisposition(),
                           std::string(),
                           download_item_->GetSuggestedFilename(),
                           GetMimeType(),
                           "download").LossyDisplayName());
}
std::string DownloadItem::GetContentDisposition() const {
  return download_item_->GetContentDisposition();
}
const GURL& DownloadItem::GetURL() const {
  return download_item_->GetURL();
}
content::DownloadItem::DownloadState DownloadItem::GetState() const {
  return download_item_->GetState();
}
bool DownloadItem::IsDone() const {
  return download_item_->IsDone();
}
void DownloadItem::SetSavePath(const base::FilePath& path) {