### 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
std::string target_id) {
  auto agent_host = content::DevToolsAgentHost::GetForId(target_id);
  WebContents* contents =
      agent_host ? WebContents::From(agent_host->GetWebContents()) : nullptr;
  return contents ? gin::CreateHandle(isolate, contents)
                  : gin::HContents>();
}
std::vector<gin::HContents>> GetAllWebContentsAsV8(
    v8::Isolate* isolate) {
  std::vector<gin::HContents>> list;
  for (auto iter = base::IDMap<WebContents*>::iterator(&GetAllWebContents());
       !iter.IsAtEnd(); iter.Advance()) {
    list.push_back(gin::CreateHandle(isolate, iter.GetCurrentValue()));
  }
  return list;
}
void Initialize(v8::Lobject> exports,
                v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context,
                void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  gin_helper::Dictionary dict(isolate, exports);
  dict.Set("WebContents", WebContents::GetConstructor(context));
  dict.SetMethod("fromId", &WebContentsFromID);
  dict.SetMethod("fromDevToolsTargetId", &WebContentsFromDevToolsTargetID);
  dict.SetMethod("getAllWebContents", &GetAllWebContentsAsV8);
}
e
NODULE_CONTEXT_AWARE(electron_browser_ontents, Initialize)