### 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
if (fullscreen == is_html_fullscreen())
    return;
  html_fullscreen_ = fullscreen;
   Notify renderer of the html fullscreen change.
  ontents()
      ->GetRenderViewHost()
      ->GetWidget()
      ->SynchronizeVisualProperties();
   The embedder WebContents is separated from the frame tree of webview, so
   we must manually sync their fullscreen states.
  if (embedder_)
    embedder_->SetHtmlApiFullscreen(fullscreen);
  if (fullscreen) {
    Emit("enter-html-full-screen");
    owner_window_->NotifyWindowEnterHtmlFullScreen();
  } else {
    Emit("leave-html-full-screen");
    owner_window_->NotifyWindowLeaveHtmlFullScreen();
  }
   Make sure all child webviews quit html fullscreen.
  if (!fullscreen && !IsGuest()) {
    auto* manager = WebViewManager::GetWebViewManager(ontents());
    manager->ForEachGuest(
        ontents(), base::BindRepeating([](content::WebContents* guest) {
          WebContents* api_ontents = WebContents::From(guest);
          api_ontents->SetHtmlApiFullscreen(false);
          return false;
        }));
  }