* Output of `node_modules/.bin/electron --version`: 4.0.0-beta.5
* Operating System (Platform and Version): macOS 10.14
* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable): There is no issue with Electron v3.

**Expected Behavior**
Cookies should be persisted in a webview that uses partition.

**Actual behavior**
Cookies are lost after relaunching app.

**To Reproduce**
1. Create webview with partition: `<webview partition="persist:webview"/>`
2. Sign in to GitHub to create a cookie.
3. Relaunch app.
4. When webview opens again, the cookies are lost. 
atom/browser/net/url_request_context_getter.cc
URLRequestContextGetter::Handle::GetResourceContext() {
  DCHECK_CURRENTLY_ON(BrowserThread::UI);
  LazyInitialize();
  return resource_context_.get();
}
scoped_refptr<URLRequestContextGetter>
URLRequestContextGetter::Handle::CreateMainRequestContextGetter(
    content::ProtocolHandlerMap* protocol_handlers,
    content::URLRequestInterceptorScopedVector protocol_interceptors) {
  DCHECK_CURRENTLY_ON(BrowserThread::UI);
  DCHECK(!main_request_context_getter_.get());
  LazyInitialize();
  main_request_context_getter_ = new URLRequestContextGetter(
      this, protocol_handlers, std::move(protocol_interceptors));
  return main_request_context_getter_;
}
scoped_refptr<URLRequestContextGetter>
URLRequestContextGetter::Handle::GetMainRequestContextGetter() {
  return main_request_context_getter_;
}
network::mojom::NetworkContextPtr
URLRequestContextGetter::Handle::GetNetworkContext() {
  if (!main_network_context_) {
    main_network_context_request_ = mojo::MakeRequest(&main_network_context_);
  }
  return std::move(main_network_context_);
}
network::mojom::NetworkContextParamsPtr
URLRequestContextGetter::Handle::CreateNetworkContextParams() {
  network::mojom::NetworkContextParamsPtr network_context_params =