* 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* context_handle,
    content::ProtocolHandlerMap* protocol_handlers,
    content::URLRequestInterceptorScopedVector protocol_interceptors)
    : context_handle_(context_handle),
      url_request_context_(nullptr),
      protocol_interceptors_(std::move(protocol_interceptors)),
      context_shutting_down_(false) {
  
  DCHECK_CURRENTLY_ON(BrowserThread::UI);
  if (protocol_handlers)
    std::swap(protocol_handlers_, *protocol_handlers);
}
URLRequestContextGetter::~URLRequestContextGetter() {
  DCHECK_CURRENTLY_ON(BrowserThread::IO);
  
  DCHECK(context_shutting_down_);
}
void URLRequestContextGetter::NotifyContextShuttingDown(
    std::unique_ptr<ResourceContext> resource_context) {
  DCHECK_CURRENTLY_ON(BrowserThread::IO);
  context_shutting_down_ = true;
  resource_context.reset();
  net::URLRequestContextGetter::NotifyContextShuttingDown();
}
net::URLRequestContext* URLRequestContextGetter::GetURLRequestContext() {