### 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
const std::string& type,
    const base::FilePath& file_system_path) {
  base::FilePath path = file_system_path;
  if (path.empty()) {
    std::vector<base::FilePath> paths;
    file_dialog::DialogSettings settings;
    settings.parent_window = owner_window();
    settings.force_detached = offscreen_;
    settings.properties = file_dialog::OPEN_DIALOG_OPEN_DIRECTORY;
    if (!file_dialog::ShowOpenDialogSync(settings, &paths))
      return;
    path = paths[0];
  }
  std::string file_system_id =
      RegisterFileSystem(GetDevToolsWebContents(), path);
  if (IsDevToolsFileSystemAdded(GetDevToolsWebContents(), path.AsUTF8Unsafe()))
    return;
  FileSystem file_system = CreateFileSystemStruct(
      GetDevToolsWebContents(), file_system_id, path.AsUTF8Unsafe(), type);
  std::unique_ptr<base::DictionaryValue> file_system_value(
      CreateFileSystemValue(file_system));
  auto* pref_service = GetPrefService(GetDevToolsWebContents());
  DictionaryPrefUpdate update(pref_service, prefs::kDevToolsFileSystemPaths);
  update.Get()->SetWithoutPathExpansion(path.AsUTF8Unsafe(),
                                        std::make_unique<base::Value>(type));
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.fileSystemAdded", nullptr, file_system_value.get(), nullptr);
}
void WebContents::DevToolsRemoveFileSystem(