### 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& content,
                                     bool save_as) {
  base::FilePath path;
  auto it = saved_files_.find(url);
  if (it != saved_files_.end() && !save_as) {
    path = it->second;
  } else {
    file_dialog::DialogSettings settings;
    settings.parent_window = owner_window();
    settings.force_detached = offscreen_;
    settings.title = url;
    settings.default_path = base::FilePath::FromUTF8Unsafe(url);
    if (!file_dialog::ShowSaveDialogSync(settings, &path)) {
      base::Value url_value(url);
      inspectable_ontents_->CallClientFunction(
          "DevToolsAPI.canceledSaveURL", &url_value, nullptr, nullptr);
      return;
    }
  }
  saved_files_[url] = path;
   Notify DevTools.
  base::Value url_value(url);
  base::Value file_system_path_value(path.AsUTF8Unsafe());
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.savedURL", &url_value, &file_system_path_value, nullptr);
  file_task_runner_->PostTask(FROM_HERE,
                              base::BindOnce(&WriteToFile, path, content));
}
void WebContents::DevToolsAppendToFile(const std::string& url,