### 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& file_system_path) {
  devtools_indexing_jobs_.erase(request_id);
  base::Value request_id_value(request_id);
  base::Value file_system_path_value(file_system_path);
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.indexingDone", &request_id_value, &file_system_path_value,
      nullptr);
}
void WebContents::OnDevToolsSearchCompleted(
    int request_id,
    const std::string& file_system_path,
    const std::vector<std::string>& file_paths) {
  base::ListValue file_paths_value;
  for (const auto& file_path : file_paths) {
    file_paths_value.Append(file_path);
  }
  base::Value request_id_value(request_id);
  base::Value file_system_path_value(file_system_path);
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.searchCompleted", &request_id_value, &file_system_path_value,
      &file_paths_value);
}
void WebContents::SetHtmlApiFullscreen(bool enter_fullscreen) {