* 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::RenderProcessHost* host,
    service_manager::mojom::ServiceRequest* service_request) {
  
  int process_id = host->GetID();
  if (IsProcessObserved(process_id))
    return;
#if BUILDFLAG(ENABLE_PRINTING)
  host->AddFilter(new printing::PrintingMessageFilter(
      process_id, host->GetBrowserContext()));
#endif
#if BUILDFLAG(ENABLE_TTS)
  host->AddFilter(new TtsMessageFilter(host->GetBrowserContext()));
#endif
  ProcessPreferences prefs;
  auto* web_preferences =
      WebContentsPreferences::From(GetWebContentsFromProcessID(process_id));
  if (web_preferences) {
    prefs.sandbox = web_preferences->IsEnabled(options::kSandbox);
    prefs.native_window_open =
        web_preferences->IsEnabled(options::kNativeWindowOpen);
    prefs.disable_popups = web_preferences->IsEnabled("disablePopups");
  }
  AddProcessPreferences(host->GetID(), prefs);
  
  host->AddObserver(this);
}
content::SpeechRecognitionManagerDelegate*