### 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* source_contents,
    int opener_render_process_id,
    int opener_render_frame_id,
    const content::mojom::CreateNewWindowParams& params,
    content::WebContents* new_contents) {
  ChildWebContentsTracker::CreateForWebContents(new_contents);
  auto* tracker = ChildWebContentsTracker::FromWebContents(new_contents);
  tracker->url = params.target_url;
  tracker->frame_name = params.frame_name;
  tracker->referrer = params.referrer.To<content::Referrer>();
  tracker->raw_features = params.raw_features;
  tracker->body = params.body;
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  gin_helper::Dictionary dict;
  gin::ConvertFromV8(isolate, pending_child_web_preferences_.Get(isolate),
                     &dict);
  pending_child_web_preferences_.Reset();
  
  
  
  
  new WebContentsPreferences(new_contents, dict);
}
bool WebContents::IsWebContentsCreationOverridden(