* 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.h
std::unique_ptr<base::Value> GetServiceManifestOverlay(
      base::StringPiece name) override;
  net::NetLog* GetNetLog() override;
  content::MediaObserver* GetMediaObserver() override;
  content::DevToolsManagerDelegate* GetDevToolsManagerDelegate() override;
  content::PlatformNotificationService* GetPlatformNotificationService()
      override;
  content::BrowserMainParts* CreateBrowserMainParts(
      const content::MainFunctionParams&) override;
  base::FilePath GetDefaultDownloadDirectory() override;
  scoped_refptr<network::SharedURLLoaderFactory>
  GetSystemSharedURLLoaderFactory() override;
  void OnNetworkServiceCreated(
      network::mojom::NetworkService* network_service) override;
  
  void RenderProcessHostDestroyed(content::RenderProcessHost* host) override;
  void RenderProcessReady(content::RenderProcessHost* host) override;
  void RenderProcessExited(
      content::RenderProcessHost* host,
      const content::ChildProcessTerminationInfo& info) override;
  bool HandleExternalProtocol(
      const GURL& url,
      content::ResourceRequestInfo::WebContentsGetter web_contents_getter,
      int child_id,
      content::NavigationUIData* navigation_data,
      bool is_main_frame,
      ui::PageTransition page_transition,
      bool has_user_gesture) override;
 private:
  struct ProcessPreferences {