* 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 content::MainFunctionParams& params) {
  return new AtomBrowserMainParts(params);
}
void AtomBrowserClient::WebNotificationAllowed(
    int render_process_id,
    const base::Callback<void(bool, bool)>& callback) {
  content::WebContents* web_contents =
      WebContentsPreferences::GetWebContentsFromProcessID(render_process_id);
  if (!web_contents) {
    callback.Run(false, false);
    return;
  }
  auto* permission_helper =
      WebContentsPermissionHelper::FromWebContents(web_contents);
  if (!permission_helper) {
    callback.Run(false, false);
    return;
  }
  permission_helper->RequestWebNotificationPermission(
      base::Bind(callback, web_contents->IsAudioMuted()));
}
void AtomBrowserClient::RenderProcessHostDestroyed(
    content::RenderProcessHost* host) {
  int process_id = host->GetID();
  pending_processes_.erase(process_id);
  RemoveProcessPreferences(process_id);
}
void AtomBrowserClient::RenderProcessReady(content::RenderProcessHost* host) {