* 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
DCHECK_CURRENTLY_ON(BrowserThread::UI);
  if (!BrowserThread::IsThreadInitialized(BrowserThread::IO) ||
      !BrowserThread::PostTask(
          BrowserThread::IO, FROM_HERE,
          base::BindOnce(&SetApplicationLocaleOnIOThread, locale))) {
    g_io_thread_application_locale.Get() = locale;
  }
  *g_application_locale = locale;
}
AtomBrowserClient::AtomBrowserClient() {
  DCHECK(!g_browser_client);
  g_browser_client = this;
}
AtomBrowserClient::~AtomBrowserClient() {
  DCHECK(g_browser_client);
  g_browser_client = nullptr;
}
content::WebContents* AtomBrowserClient::GetWebContentsFromProcessID(
    int process_id) {
  
  
  if (base::ContainsKey(pending_processes_, process_id))
    return pending_processes_[process_id];
  
  
  return WebContentsPreferences::GetWebContentsFromProcessID(process_id);
}
bool AtomBrowserClient::ShouldCreateNewSiteInstance(