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### Preflight Checklist
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* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**  9.0.0
  <!-- (output of `node_modules/.bin/electron --version`) e.g. 4.0.3 -->
* **Operating System:** Windows 10 (1904)
  <!-- (Platform and Version) e.g. macOS 10.13.6 / Windows 10 (1803) / Ubuntu 18.04 x64 -->
* **Last Known Working Electron version:** 8.2.0
  <!-- (if applicable) e.g. 3.1.0 -->

### Expected Behavior
<!-- A clear and concise description of what you expected to happen. -->
Setting BrowserWindow webPreferences: { webSecurity: false } should disable the CORS policy

### Actual Behavior
<!-- A clear and concise description of what actually happened. -->
Although I am receiving the electron security warnings for "Disabled webSecurity","allowRunningInsecureContent" and "Insecure Content-Security-Policy" in the console I am still getting COR policy errors on my requests.

**Here is an example:**

Access to XMLHttpRequest at 'http://localhost:8080/users/login' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

shell/browser/electron_browser_client.cc
prefs->application_cache_enabled = true;
  prefs->allow_universal_access_from_file_urls = true;
  prefs->allow_file_access_from_file_urls = true;
  prefs->webgl1_enabled = true;
  prefs->webgl2_enabled = true;
  prefs->allow_running_insecure_content = false;
  prefs->default_minimum_page_scale_factor = 1.f;
  prefs->default_maximum_page_scale_factor = 1.f;
  prefs->navigate_on_drag_drop = false;
#if !BUILDFLAG(ENABLE_PICTURE_IN_PICTURE)
  prefs->picture_in_picture_enabled = false;
#endif
  ui::NativeTheme* native_theme = ui::NativeTheme::GetInstanceForNativeUi();
  prefs->preferred_color_scheme = native_theme->ShouldUseDarkColors()
                                      ? blink::PreferredColorScheme::kDark
                                      : blink::PreferredColorScheme::kLight;
  SetFontDefaults(prefs);
  
  auto* web_contents = content::WebContents::FromRenderViewHost(host);
  auto* web_preferences = WebContentsPreferences::From(web_contents);
  if (web_preferences)
    web_preferences->OverrideWebkitPrefs(prefs);
}
void ElectronBrowserClient::SetCanUseCustomSiteInstance(bool should_disable) {
  disable_process_restart_tricks_ = should_disable;
}
bool ElectronBrowserClient::CanUseCustomSiteInstance() {
  return disable_process_restart_tricks_;
}
content::ContentBrowserClient::SiteInstanceForNavigationType