### 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
script_executor_ = std::make_unique<extensions::ScriptExecutor>(ontents);
#endif
  auto session = Session::CreateFrom(isolate, GetBrowserContext());
  session_.Reset(isolate, session.ToV8());
  ontents->SetUserAgentOverride(blink::UserAgentOverride::UserAgentOnly(
                                         GetBrowserContext()->GetUserAgent()),
                                     false);
  ontents->SetUserData(kElectronApiWebContentsKey,
                            std::make_unique<UserDataLink>(GetWeakPtr()));
  InitZoomController(ontents, gin::Dictionary::CreateEmpty(isolate));
}
WebContents::WebContents(v8::Isolate* isolate,
                         std::unique_ptr<content::WebContents> ontents,
                         Type type)
    : content::WebContentsObserver(ontents.get()),
      type_(type),
      id_(GetAllWebContents().Add(this)),
      devtools_file_system_indexer_(
          base::MakeRefCounted<DevToolsFileSystemIndexer>()),
      exclusive_access_manager_(std::make_unique<ExclusiveAccessManager>(this)),
      file_task_runner_(
          base::ThreadPool::CreateSequencedTaskRunner({base::MayBlock()}))
#if BUILDFLAG(ENABLE_PRINTING)
      ,
      print_task_runner_(CreatePrinterHandlerTaskRunner())
#endif
{
  DCHECK(type != Type::kRemote)
      << "Can't take ownership of a remote WebContents";
  auto session = Session::CreateFrom(isolate, GetBrowserContext());