- 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_session.cc
BrowserThread::PostTask(
      BrowserThread::UI, FROM_HERE, base::Bind(callback, result...));
}
void OnGetBackend(disk_cache::Backend** backend_ptr,
                  Session::CacheAction action,
                  const net::CompletionCallback& callback,
                  int result) {
  if (result != net::OK) {
    RunCallbackInUI(callback, result);
  } else if (backend_ptr && *backend_ptr) {
    if (action == Session::CacheAction::CLEAR) {
      (*backend_ptr)->DoomAllEntries(base::Bind(&RunCallbackInUI<int>,
                                                callback));
    } else if (action == Session::CacheAction::STATS) {
      base::StringPairs stats;
      (*backend_ptr)->GetStats(&stats);
      for (const auto& stat : stats) {
        if (stat.first == "Current size") {
          int current_size;
          base::StringToInt(stat.second, &current_size);
          RunCallbackInUI(callback, current_size);
          break;
        }
      }
    }
  } else {
    RunCallbackInUI<int>(callback, net::ERR_FAILED);
  }
}
void DoCacheActionInIO(