### 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 base::FilePath& path) {
  auto* isolated_context = storage::IsolatedContext::GetInstance();
  std::string root_name(kRootName);
  storage::IsolatedContext::ScopedFSHandle file_system =
      isolated_context->RegisterFileSystemForPath(
          storage::kFileSystemTypeLocal, std::string(), path, &root_name);
  content::ChildProcessSecurityPolicy* policy =
      content::ChildProcessSecurityPolicy::GetInstance();
  content::RenderViewHost* render_view_host = ontents->GetRenderViewHost();
  int renderer_id = render_view_host->GetProcess()->GetID();
  policy->GrantReadFileSystem(renderer_id, file_system.id());
  policy->GrantWriteFileSystem(renderer_id, file_system.id());
  policy->GrantCreateFileForFileSystem(renderer_id, file_system.id());
  policy->GrantDeleteFromFileSystem(renderer_id, file_system.id());
  if (!policy->CanReadFile(renderer_id, path))
    policy->GrantReadFile(renderer_id, path);
  return file_system.id();
}
FileSystem CreateFileSystemStruct(content::WebContents* ontents,