* 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
OutOfProcessServiceMap* services) {
  (*services)[proxy_resolver::mojom::kProxyResolverServiceName] =
      base::BindRepeating(&l10n_util::GetStringUTF16,
                          IDS_UTILITY_PROCESS_PROXY_RESOLVER_NAME);
#if BUILDFLAG(ENABLE_PRINTING)
  (*services)[printing::mojom::kServiceName] =
      base::BindRepeating(&l10n_util::GetStringUTF16,
                          IDS_UTILITY_PROCESS_PDF_COMPOSITOR_SERVICE_NAME);
  (*services)[printing::mojom::kChromePrintingServiceName] =
      base::BindRepeating(&l10n_util::GetStringUTF16,
                          IDS_UTILITY_PROCESS_PRINTING_SERVICE_NAME);
#endif
}
std::unique_ptr<base::Value> AtomBrowserClient::GetServiceManifestOverlay(
    base::StringPiece name) {
  ui::ResourceBundle& rb = ui::ResourceBundle::GetSharedInstance();
  int id = -1;
  if (name == content::mojom::kBrowserServiceName)
    id = IDR_ELECTRON_CONTENT_BROWSER_MANIFEST_OVERLAY;
  else if (name == content::mojom::kPackagedServicesServiceName)
    id = IDR_ELECTRON_CONTENT_PACKAGED_SERVICES_MANIFEST_OVERLAY;
  if (id == -1)
    return nullptr;
  base::StringPiece manifest_contents = rb.GetRawDataResource(id);
  return base::JSONReader::Read(manifest_contents);
}
net::NetLog* AtomBrowserClient::GetNetLog() {
  return g_browser_process->net_log();
}
content::BrowserMainParts* AtomBrowserClient::CreateBrowserMainParts(