* 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
command_line->AppendSwitchASCII(switches::kRegisterServiceWorkerSchemes,
                                    *g_custom_service_worker_schemes);
#if defined(OS_WIN)
  
  PWSTR current_app_id;
  if (SUCCEEDED(GetCurrentProcessExplicitAppUserModelID(&current_app_id))) {
    command_line->AppendSwitchNative(switches::kAppUserModelId, current_app_id);
    CoTaskMemFree(current_app_id);
  }
#endif
  if (delegate_) {
    auto app_path = static_cast<api::App*>(delegate_)->GetAppPath();
    command_line->AppendSwitchPath(switches::kAppPath, app_path);
  }
  content::WebContents* web_contents = GetWebContentsFromProcessID(process_id);
  if (web_contents) {
    auto* web_preferences = WebContentsPreferences::From(web_contents);
    if (web_preferences)
      web_preferences->AppendCommandLineSwitches(command_line);
    SessionPreferences::AppendExtraCommandLineSwitches(
        web_contents->GetBrowserContext(), command_line);
  }
}
void AtomBrowserClient::DidCreatePpapiPlugin(content::BrowserPpapiHost* host) {
#if BUILDFLAG(ENABLE_PEPPER_FLASH)
  host->GetPpapiHost()->AddHostFactoryFilter(
      base::WrapUnique(new ChromeBrowserPepperHostFactory(host)));
#endif
}
std::string AtomBrowserClient::GetGeolocationApiKey() {