* 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
DCHECK_CURRENTLY_ON(BrowserThread::UI);
  if (initialized_)
    return;
  initialized_ = true;
  main_network_context_params_ = CreateNetworkContextParams();
  browser_context_->proxy_config_monitor()->AddToNetworkContextParams(
      main_network_context_params_.get());
  BrowserProcessImpl::ApplyProxyModeFromCommandLine(
      browser_context_->in_memory_pref_store());
  if (!main_network_context_request_.is_pending()) {
    main_network_context_request_ = mojo::MakeRequest(&main_network_context_);
  }
  content::BrowserContext::EnsureResourceContextInitialized(
      browser_context_.get());
}
void URLRequestContextGetter::Handle::ShutdownOnUIThread() {
  DCHECK_CURRENTLY_ON(BrowserThread::UI);
  if (main_request_context_getter_.get()) {
    if (BrowserThread::IsThreadInitialized(BrowserThread::IO)) {
      BrowserThread::PostTask(
          BrowserThread::IO, FROM_HERE,
          base::BindOnce(&URLRequestContextGetter::NotifyContextShuttingDown,
                         base::RetainedRef(main_request_context_getter_),
                         std::move(resource_context_)));
    }
  }
  if (!BrowserThread::DeleteSoon(BrowserThread::IO, FROM_HERE, this))
    delete this;
}
URLRequestContextGetter::URLRequestContextGetter(