- 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
mate::Arguments* args) {
  browser_context_->SetUserAgent(user_agent);
  std::string accept_lang = l10n_util::GetApplicationLocale("");
  args->GetNext(&accept_lang);
  auto getter = browser_context_->GetRequestContext();
  getter->GetNetworkTaskRunner()->PostTask(
      FROM_HERE,
      base::Bind(&SetUserAgentInIO, getter, accept_lang, user_agent));
}
std::string Session::GetUserAgent() {
  return browser_context_->GetUserAgent();
}
void Session::GetBlobData(
    const std::string& uuid,
    const AtomBlobReader::CompletionCallback& callback) {
  if (callback.is_null())
    return;
  AtomBlobReader* blob_reader =
      browser_context()->GetBlobReader();
  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
      base::Bind(&AtomBlobReader::StartReading,
                 base::Unretained(blob_reader),
                 uuid,
                 callback));
}
v8::Local<v8::Value> Session::Cookies(v8::Isolate* isolate) {