- 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
if (!callback.is_null())
    callback.Run();
}
}  
Session::Session(v8::Isolate* isolate, AtomBrowserContext* browser_context)
    : devtools_network_emulation_client_id_(base::GenerateGUID()),
      browser_context_(browser_context) {
  
  content::BrowserContext::GetDownloadManager(browser_context)->
      AddObserver(this);
  Init(isolate);
  AttachAsUserData(browser_context);
}
Session::~Session() {
  content::BrowserContext::GetDownloadManager(browser_context())->
      RemoveObserver(this);
  g_sessions.erase(weak_map_id());
}
void Session::OnDownloadCreated(content::DownloadManager* manager,