- 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
const GURL& url) {
    DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
    net::ProxyService* proxy_service =
        context_getter->GetURLRequestContext()->proxy_service();
    net::CompletionCallback completion_callback =
        base::Bind(&ResolveProxyHelper::OnResolveProxyCompleted,
                   base::Unretained(this));
    
    int result = proxy_service->ResolveProxy(
        url, "GET", net::LOAD_NORMAL, &proxy_info_, completion_callback,
        &pac_req_, nullptr, net::BoundNetLog());
    
    if (result != net::ERR_IO_PENDING)
      completion_callback.Run(result);
  }
  Session::ResolveProxyCallback callback_;
  net::ProxyInfo proxy_info_;
  net::ProxyService::PacRequest* pac_req_;
  scoped_refptr<base::SingleThreadTaskRunner> original_thread_;
  DISALLOW_COPY_AND_ASSIGN(ResolveProxyHelper);
};
template<typename ...T>
void RunCallbackInUI(const base::Callback<void(T...)>& callback, T... result) {