* 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
render_process_host_pids_[host->GetID()] =
      base::GetProcId(host->GetProcess().Handle());
  if (delegate_) {
    static_cast<api::App*>(delegate_)->RenderProcessReady(host);
  }
}
void AtomBrowserClient::RenderProcessExited(
    content::RenderProcessHost* host,
    const content::ChildProcessTerminationInfo& info) {
  auto host_pid = render_process_host_pids_.find(host->GetID());
  if (host_pid != render_process_host_pids_.end()) {
    if (delegate_) {
      static_cast<api::App*>(delegate_)->RenderProcessDisconnected(
          host_pid->second);
    }
    render_process_host_pids_.erase(host_pid);
  }
}
void OnOpenExternal(const GURL& escaped_url, bool allowed) {
  if (allowed)
    platform_util::OpenExternal(
#if defined(OS_WIN)
        base::UTF8ToUTF16(escaped_url.spec()),
#else
        escaped_url,
#endif
        platform_util::OpenExternalOptions());
}
void HandleExternalProtocolInUI(