### 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
auto file_system_paths = GetAddedFileSystemPaths(GetDevToolsWebContents());
  if (file_system_paths.empty()) {
    base::ListValue empty_file_system_value;
    inspectable_ontents_->CallClientFunction(
        "DevToolsAPI.fileSystemsLoaded", &empty_file_system_value, nullptr,
        nullptr);
    return;
  }
  std::vector<FileSystem> file_systems;
  for (const auto& file_system_path : file_system_paths) {
    base::FilePath path =
        base::FilePath::FromUTF8Unsafe(file_system_path.first);
    std::string file_system_id =
        RegisterFileSystem(GetDevToolsWebContents(), path);
    FileSystem file_system =
        CreateFileSystemStruct(GetDevToolsWebContents(), file_system_id,
                               file_system_path.first, file_system_path.second);
    file_systems.push_back(file_system);
  }
  base::ListValue file_system_value;
  for (const auto& file_system : file_systems)
    file_system_value.Append(CreateFileSystemValue(file_system));
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.fileSystemsLoaded", &file_system_value, nullptr, nullptr);
}
void WebContents::DevToolsAddFileSystem(