### 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
int request_id,
    const std::string& file_system_path,
    const std::string& excluded_folders_message) {
  if (!IsDevToolsFileSystemAdded(GetDevToolsWebContents(), file_system_path)) {
    OnDevToolsIndexingDone(request_id, file_system_path);
    return;
  }
  if (devtools_indexing_jobs_.count(request_id) != 0)
    return;
  std::vector<std::string> excluded_folders;
  std::unique_ptr<base::Value> parsed_excluded_folders =
      base::JSONReader::ReadDeprecated(excluded_folders_message);
  if (parsed_excluded_folders && parsed_excluded_folders->is_list()) {
    for (const base::Value& folder_path : parsed_excluded_folders->GetList()) {
      if (folder_path.is_string())
        excluded_folders.push_back(folder_path.GetString());
    }
  }
  devtools_indexing_jobs_[request_id] =
      scoped_refptr<DevToolsFileSystemIndexer::FileSystemIndexingJob>(
          devtools_file_system_indexer_->IndexPath(
              file_system_path, excluded_folders,
              base::BindRepeating(
                  &WebContents::OnDevToolsIndexingWorkCalculated,