* Output of `node_modules/.bin/electron --version`: **v4.0.0-beta.3**
* Operating System (Platform and Version): **Windows 10**
* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable): **v3.0.4**

**Expected Behavior**
With electron 3, I could pass `webPreferences: {webwebSecurity: false}` to the BrowserWindow constructor (the equivalent of the `--disable-web-security` Chrome command line argument) which would disable CORB (effectively allowing cross-origin requests).

**Actual behavior**
CORB is still doing its thing (ignoring the `{webwebSecurity: false}` option; I see entries like these in the console:
`Cross-Origin Read Blocking (CORB) blocked cross-origin response <URL> with MIME type application/json. See <URL> for more details.`

**To Reproduce**
Make a request (using fetch or xhr) to a file on a different domain.
Example repo:
```
git clone  https://github.com/IAmPicard/electron-cors-repro
npm install
npm run start
```


atom/browser/atom_browser_client.cc
base::FilePath path;
  if (base::PathService::Get(base::DIR_HOME, &path))
    path = path.Append(FILE_PATH_LITERAL("Downloads"));
  return path;
}
scoped_refptr<network::SharedURLLoaderFactory>
AtomBrowserClient::GetSystemSharedURLLoaderFactory() {
  if (!g_browser_process)
    return nullptr;
  return g_browser_process->shared_url_loader_factory();
}
void AtomBrowserClient::OnNetworkServiceCreated(
    network::mojom::NetworkService* network_service) {
  if (!g_browser_process)
    return;
  g_browser_process->system_network_context_manager()->OnNetworkServiceCreated(
      network_service);
}
std::string AtomBrowserClient::GetApplicationLocale() {
  if (BrowserThread::CurrentlyOn(BrowserThread::IO))
    return g_io_thread_application_locale.Get();
  return *g_application_locale;
}
}