### 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
std::unique_ptr<content::WebContents> ontents,
    ElectronBrowserContext* browser_context,
    bool is_guest) {
  browser_context_ = browser_context;
  ontents->SetDelegate(this);
#if BUILDFLAG(ENABLE_PRINTING)
  PrintPreviewMessageHandler::CreateForWebContents(ontents.get());
  PrintViewManagerElectron::CreateForWebContents(ontents.get());
  printing::CreateCompositeClientIfNeeded(ontents.get(),
                                          browser_context->GetUserAgent());
#endif
#if BUILDFLAG(ENABLE_PDF_VIEWER)
  pdf::PDFWebContentsHelper::CreateForWebContentsWithClient(
      ontents.get(),
      std::make_unique<ElectronPDFWebContentsHelperClient>());
#endif
  
  auto* web_preferences = WebContentsPreferences::From(ontents.get());
  offscreen_ = web_preferences && web_preferences->IsOffscreen();
  
  inspectable_ontents_ = std::make_unique<InspectableWebContents>(
      std::move(ontents), browser_context->prefs(), is_guest);
  inspectable_ontents_->SetDelegate(this);
}
WebContents::~WebContents() {