### Preflight Checklist

- [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/main/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [X] I have searched the [issue tracker](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Electron Version

14.0.1

### What operating system are you using?

macOS

### Operating System Version

macOS Big Sur 11.6.1

### What arch are you using?

x64

### Last Known Working Electron version

14.0.0

### Expected Behavior


When `BrowserView.setBackgroundColor` is called with an explicit color, expect the background to be the color selected.

When `BrowserView.setBackgroundColor` is not called, expect the background of the `BrowserView` to be transparent by default; and can see through `BrowserWindow`'s background color.



### Actual Behavior

The `BrowserView` background is always white.

### Testcase Gist URL

https://gist.github.com/kevinlinv/2de60478ffec6a2f18f1ec0fc6f5add9

### Additional Information

Here is a quick recording that reproduces the error.

https://user-images.githubusercontent.com/44967897/140694174-e27aff28-1847-4813-bc46-d9fda6569128.mp4


shell/browser/api/electron_api_web_contents.cc
content::WebContents* ontents,
    const content::MediaStreamRequest& request,
    content::MediaResponseCallback callback) {
  auto* permission_helper =
      WebContentsPermissionHelper::FromWebContents(ontents);
  permission_helper->RequestMediaAccessPermission(request, std::move(callback));
}
void WebContents::RequestToLockMouse(content::WebContents* ontents,
                                     bool user_gesture,
                                     bool last_unlocked_by_target) {
  auto* permission_helper =
      WebContentsPermissionHelper::FromWebContents(ontents);
  permission_helper->RequestPointerLockPermission(user_gesture);
}
content::JavaScriptDialogManager* WebContents::GetJavaScriptDialogManager(
    content::WebContents* source) {
  if (!dialog_manager_)
    dialog_manager_ = std::make_unique<ElectronJavaScriptDialogManager>();
  return dialog_manager_.get();
}
void WebContents::OnAudioStateChanged(bool audible) {
  Emit("-audio-state-changed", audible);
}
void WebContents::BeforeUnloadFired(bool proceed,