* 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
const GURL& src_url,
                   const GURL& dest_url) {
  return content::SiteInstance::IsSameWebSite(browser_context, src_url,
                                              dest_url) ||
         
         
         
         content::SiteInstance::GetSiteForURL(browser_context, dest_url) ==
             src_url;
}
AtomBrowserClient* g_browser_client = nullptr;
base::LazyInstance<std::string>::DestructorAtExit
    g_io_thread_application_locale = LAZY_INSTANCE_INITIALIZER;
base::NoDestructor<std::string> g_application_locale;
void SetApplicationLocaleOnIOThread(const std::string& locale) {
  DCHECK_CURRENTLY_ON(BrowserThread::IO);
  g_io_thread_application_locale.Get() = locale;
}
}  
void AtomBrowserClient::SuppressRendererProcessRestartForOnce() {
  g_suppress_renderer_process_restart = true;
}
void AtomBrowserClient::SetCustomServiceWorkerSchemes(
    const std::vector<std::string>& schemes) {
  *g_custom_service_worker_schemes = base::JoinString(schemes, ",");
}
AtomBrowserClient* AtomBrowserClient::Get() {
  return g_browser_client;
}
void AtomBrowserClient::SetApplicationLocale(const std::string& locale) {