* 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
class PlatformNotificationService;
class AtomBrowserClient : public content::ContentBrowserClient,
                          public content::RenderProcessHostObserver {
 public:
  static AtomBrowserClient* Get();
  static void SetApplicationLocale(const std::string& locale);
  AtomBrowserClient();
  ~AtomBrowserClient() override;
  using Delegate = content::ContentBrowserClient;
  void set_delegate(Delegate* delegate) { delegate_ = delegate; }
  
  content::WebContents* GetWebContentsFromProcessID(int process_id);
  
  static void SuppressRendererProcessRestartForOnce();
  
  static void SetCustomServiceWorkerSchemes(
      const std::vector<std::string>& schemes);
  NotificationPresenter* GetNotificationPresenter();
  void WebNotificationAllowed(int render_process_id,
                              const base::Callback<void(bool, bool)>& callback);
  
  std::vector<std::unique_ptr<content::NavigationThrottle>>
  CreateThrottlesForNavigation(content::NavigationHandle* handle) override;
  
  std::string GetApplicationLocale() override;
 protected:
  void RenderProcessWillLaunch(
      content::RenderProcessHost* host,
      service_manager::mojom::ServiceRequest* service_request) override;
  content::SpeechRecognitionManagerDelegate*