- 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 (cookies_.IsEmpty()) {
    auto handle = Cookies::Create(isolate, browser_context());
    cookies_.Reset(isolate, handle.ToV8());
  }
  return v8::Local<v8::Value>::New(isolate, cookies_);
}
v8::Local<v8::Value> Session::Protocol(v8::Isolate* isolate) {
  if (protocol_.IsEmpty()) {
    auto handle = atom::api::Protocol::Create(isolate, browser_context());
    protocol_.Reset(isolate, handle.ToV8());
  }
  return v8::Local<v8::Value>::New(isolate, protocol_);
}
v8::Local<v8::Value> Session::WebRequest(v8::Isolate* isolate) {
  if (web_request_.IsEmpty()) {
    auto handle = atom::api::WebRequest::Create(isolate, browser_context());
    web_request_.Reset(isolate, handle.ToV8());
  }
  return v8::Local<v8::Value>::New(isolate, web_request_);
}
mate::Handle<Session> Session::CreateFrom(