- 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
content::DownloadItem* download_item)
    : download_item_(download_item) {
  download_item_->AddObserver(this);
  Init(isolate);
  AttachAsUserData(download_item);
}
DownloadItem::~DownloadItem() {
  if (download_item_) {
    
    download_item_->RemoveObserver(this);
    download_item_->Remove();
  }
  
  g_download_item_objects.erase(weak_map_id());
}
void DownloadItem::OnDownloadUpdated(content::DownloadItem* item) {
  if (download_item_->IsDone()) {
    Emit("done", item->GetState());
    
    base::MessageLoop::current()->PostTask(FROM_HERE, GetDestroyClosure());
  } else {
    Emit("updated", item->GetState());
  }
}
void DownloadItem::OnDownloadDestroyed(content::DownloadItem* download_item) {