### 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
content::WebContents* source,
    const content::OpenURLParams& params) {
  auto weak_this = GetWeakPtr();
  if (params.disposition != WindowOpenDisposition::CURRENT_TAB) {
    Emit("-new-window", params.url, "", params.disposition, "", params.referrer,
         params.post_data);
    return nullptr;
  }
  if (!weak_this || !ontents())
    return nullptr;
  content::NavigationController::LoadURLParams load_url_params(params.url);
  load_url_params.referrer = params.referrer;
  load_url_params.transition_type = params.transition;
  load_url_params.extra_headers = params.extra_headers;
  load_url_params.should_replace_current_entry =
      params.should_replace_current_entry;
  load_url_params.is_renderer_initiated = params.is_renderer_initiated;
  load_url_params.started_from_context_menu = params.started_from_context_menu;
  load_url_params.initiator_origin = params.initiator_origin;
  load_url_params.source_site_instance = params.source_site_instance;
  load_url_params.frame_tree_node_id = params.frame_tree_node_id;