* 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
AtomBrowserClient::CreateSpeechRecognitionManagerDelegate() {
  return new AtomSpeechRecognitionManagerDelegate;
}
void AtomBrowserClient::OverrideWebkitPrefs(content::RenderViewHost* host,
                                            content::WebPreferences* prefs) {
  prefs->javascript_enabled = true;
  prefs->web_security_enabled = true;
  prefs->plugins_enabled = true;
  prefs->dom_paste_enabled = true;
  prefs->allow_scripts_to_close_windows = true;
  prefs->javascript_can_access_clipboard = true;
  prefs->local_storage_enabled = true;
  prefs->databases_enabled = true;
  prefs->application_cache_enabled = true;
  prefs->allow_universal_access_from_file_urls = true;
  prefs->allow_file_access_from_file_urls = true;
  prefs->webgl1_enabled = true;
  prefs->webgl2_enabled = true;
  prefs->allow_running_insecure_content = false;
  prefs->default_minimum_page_scale_factor = 1.f;
  prefs->default_maximum_page_scale_factor = 1.f;
  prefs->navigate_on_drag_drop = false;
  SetFontDefaults(prefs);
  
  auto* web_contents = content::WebContents::FromRenderViewHost(host);
  auto* web_preferences = WebContentsPreferences::From(web_contents);
  if (web_preferences)
    web_preferences->OverrideWebkitPrefs(prefs);
}
void AtomBrowserClient::OverrideSiteInstanceForNavigation(