- 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) {
  AtomPermissionManager::RequestHandler handler;
  if (!(val->IsNull() || mate::ConvertFromV8(args->isolate(), val, &handler))) {
    args->ThrowError("Must pass null or function");
    return;
  }
  auto permission_manager = static_cast<AtomPermissionManager*>(
      browser_context()->GetPermissionManager());
  permission_manager->SetPermissionRequestHandler(handler);
}
void Session::ClearHostResolverCache(mate::Arguments* args) {
  base::Closure callback;
  args->GetNext(&callback);
  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
      base::Bind(&ClearHostResolverCacheInIO,
                 make_scoped_refptr(browser_context_->GetRequestContext()),
                 callback));
}
void Session::AllowNTLMCredentialsForDomains(const std::string& domains) {
  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
      base::Bind(&AllowNTLMCredentialsForDomainsInIO,
                 make_scoped_refptr(browser_context_->GetRequestContext()),
                 domains));
}
void Session::SetUserAgent(const std::string& user_agent,