### 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
void WebContents::UpdateExclusiveAccessExitBubbleContent(
    const GURL& url,
    ExclusiveAccessBubbleType bubble_type,
    ExclusiveAccessBubbleHideCallback bubble_first_hide_callback,
    bool force_update) {}
void WebContents::OnExclusiveAccessUserInput() {}
content::WebConbContents::GetActiveWebContents() {
  return ontents();
}
bool WebContents::CanUserExitFullscreen() const {
  return true;
}
bool WebContents::IsExclusiveAccessBubbleDisplayed() const {
  return false;
}
void WebContents::EnterFullscreenModeForTab(
    content::RenderFrameHost* requesting_frame,
    const blink::mojom::FullscreenOptions& options) {
  auto* source = content::WebContents::FromRenderFrameHost(requesting_frame);
  auto* permission_helper =
      WebContentsPermissionHelper::FromWebContents(source);
  auto callback =
      base::BindRepeating(&WebContents::OnEnterFullscreenModeForTab,
                          base::Unretained(this), requesting_frame, options);
  permission_helper->RequestFullscreenPermission(callback);
  exclusive_access_manager_->fullscreen_controller()->EnterFullscreenModeForTab(
      requesting_frame, options.display_id);
}
void WebContents::OnEnterFullscreenModeForTab(