* 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
IsSameWebSite(browser_context, iter->second->GetSiteURL(), dest_site)) {
      *new_instance = iter->second;
    } else {
      site_per_affinities[affinity] = candidate_instance;
      *new_instance = candidate_instance;
      
      auto* pending_process = candidate_instance->GetProcess();
      pending_processes_[pending_process->GetID()] = web_contents;
    }
  } else {
    
    
    
    
    
    
    
    
    
    if (has_request_started) {
      *new_instance = current_instance;
      return;
    }
    *new_instance = candidate_instance;
    
    auto* pending_process = candidate_instance->GetProcess();
    pending_processes_[pending_process->GetID()] = web_contents;
  }
}
void AtomBrowserClient::AppendExtraCommandLineSwitches(