* 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
CreateSpeechRecognitionManagerDelegate() override;
  void OverrideWebkitPrefs(content::RenderViewHost* render_view_host,
                           content::WebPreferences* prefs) override;
  void OverrideSiteInstanceForNavigation(
      content::RenderFrameHost* render_frame_host,
      content::BrowserContext* browser_context,
      const GURL& dest_url,
      bool has_request_started,
      content::SiteInstance* candidate_instance,
      content::SiteInstance** new_instance) override;
  void AppendExtraCommandLineSwitches(base::CommandLine* command_line,
                                      int child_process_id) override;
  void DidCreatePpapiPlugin(content::BrowserPpapiHost* browser_host) override;
  std::string GetGeolocationApiKey() override;
  content::QuotaPermissionContext* CreateQuotaPermissionContext() override;
  void AllowCertificateError(
      content::WebContents* web_contents,
      int cert_error,
      const net::SSLInfo& ssl_info,
      const GURL& request_url,
      content::ResourceType resource_type,
      bool strict_enforcement,
      bool expired_previous_decision,
      const base::Callback<void(content::CertificateRequestResultType)>&
          callback) override;
  void SelectClientCertificate(
      content::WebContents* web_contents,
      net::SSLCertRequestInfo* cert_request_info,
      net::ClientCertIdentityList client_certs,
      std::unique_ptr<content::ClientCertificateDelegate> delegate) override;