* 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
content::WebContents* web_contents,
    net::SSLCertRequestInfo* cert_request_info,
    net::ClientCertIdentityList client_certs,
    std::unique_ptr<content::ClientCertificateDelegate> delegate) {
  if (!client_certs.empty() && delegate_) {
    delegate_->SelectClientCertificate(web_contents, cert_request_info,
                                       std::move(client_certs),
                                       std::move(delegate));
  }
}
void AtomBrowserClient::ResourceDispatcherHostCreated() {
  resource_dispatcher_host_delegate_.reset(
      new AtomResourceDispatcherHostDelegate);
  content::ResourceDispatcherHost::Get()->SetDelegate(
      resource_dispatcher_host_delegate_.get());
}
bool AtomBrowserClient::CanCreateWindow(
    content::RenderFrameHost* opener,
    const GURL& opener_url,
    const GURL& opener_top_level_frame_url,
    const GURL& source_origin,
    content::mojom::WindowContainerType container_type,
    const GURL& target_url,
    const content::Referrer& referrer,
    const std::string& frame_name,
    WindowOpenDisposition disposition,
    const blink::mojom::WindowFeatures& features,
    const std::vector<std::string>& additional_features,