### 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
}
 static
v8::LobjectTemplate> WebContents::FillObjectTemplate(
    v8::Isolate* isolate,
    v8::LobjectTemplate> templ) {
  gin::InvokerOptions options;
  options.holder_is_first_argument = true;
  options.holder_type = "WebContents";
  templ->Set(
      gin::StringToSymbol(isolate, "isDestroyed"),
      gin::CreateFunctionTemplate(
          isolate, base::BindRepeating(&gin_helper::Destroyable::IsDestroyed),
          options));
   We use gin_helper::ObjectTemplateBuilder instead of
   gin::ObjectTemplateBuilder here to handle the fact that WebContents is
   destroyable.
  lper::ObjectTemplateBuilder(isolate, templ)
      .SetMethod("destroy", &WebContents::Destroy)
      .SetMethod("getBackgroundThrottling",
                 &WebContents::GetBackgroundThrottling)
      .SetMethod("setBackgroundThrottling",
                 &WebContents::SetBackgroundThrottling)
      .SetMethod("getProcessId", &WebContents::GetProcessID)
      .SetMethod("getOSProcessId", &WebContents::GetOSProcessID)
      .SetMethod("equal", &WebContents::Equal)
      .SetMethod("_loadURL", &WebContents::LoadURL)
      .SetMethod("reload", &WebContents::Reload)
      .SetMethod("reloadIgnoringCache", &WebContents::ReloadIgnoringCache)
      .SetMethod("downloadURL", &WebContents::DownloadURL)
      .SetMethod("getURL", &WebContents::GetURL)