* 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
#include "atom/browser/api/atom_api_app.h"
#include "atom/browser/api/atom_api_protocol.h"
#include "atom/browser/api/atom_api_web_contents.h"
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/atom_browser_main_parts.h"
#include "atom/browser/atom_navigation_throttle.h"
#include "atom/browser/atom_quota_permission_context.h"
#include "atom/browser/atom_resource_dispatcher_host_delegate.h"
#include "atom/browser/atom_speech_recognition_manager_delegate.h"
#include "atom/browser/child_web_contents_tracker.h"
#include "atom/browser/font_defaults.h"
#include "atom/browser/io_thread.h"
#include "atom/browser/media/media_capture_devices_dispatcher.h"
#include "atom/browser/native_window.h"
#include "atom/browser/notifications/notification_presenter.h"
#include "atom/browser/notifications/platform_notification_service.h"
#include "atom/browser/session_preferences.h"
#include "atom/browser/ui/devtools_manager_delegate.h"
#include "atom/browser/web_contents_permission_helper.h"
#include "atom/browser/web_contents_preferences.h"
#include "atom/browser/window_list.h"
#include "atom/common/options_switches.h"
#include "atom/common/platform_util.h"
#include "base/command_line.h"
#include "base/environment.h"
#include "base/files/file_util.h"
#include "base/json/json_reader.h"
#include "base/lazy_instance.h"
#include "base/no_destructor.h"
#include "base/path_service.h"