- 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
download_item_ = nullptr;
  
  delete this;
}
void DownloadItem::Pause() {
  download_item_->Pause();
}
bool DownloadItem::IsPaused() const {
  return download_item_->IsPaused();
}
void DownloadItem::Resume() {
  download_item_->Resume();
}
bool DownloadItem::CanResume() const {
  return download_item_->CanResume();
}
void DownloadItem::Cancel() {
  download_item_->Cancel(true);
  download_item_->Remove();
}
int64_t DownloadItem::GetReceivedBytes() const {
  return download_item_->GetReceivedBytes();
}
int64_t DownloadItem::GetTotalBytes() const {