### 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
TRACE_EVENT1("electron", "WebContents::MessageHost", "channel", channel);
   webContents.emit('ipc-message-host', new Event(), channel, args);
  EmitWithSender("ipc-message-host", render_frame_host,
                 electron::mojom::ElectronBrowser::InvokeCallback(), channel,
                 std::move(arguments));
}
void WebContents::UpdateDraggableRegions(
    std::vector<mojom::DraggableRegionPtr> regions) {
  for (ExtendedWebContentsObserver& observer : observers_)
    observer.OnDraggableRegionsUpdated(regions);
}
void WebContents::DidStartNavigation(
    content::NavigationHandle* navigation_handle) {
  EmitNavigationEvent("did-start-navigation", navigation_handle);
}
void WebContents::DidRedirectNavigation(
    content::NavigationHandle* navigation_handle) {
  EmitNavigationEvent("did-redirect-navigation", navigation_handle);
}
void WebContents::ReadyToCommitNavigation(
    content::NavigationHandle* navigation_handle) {
   Don't focus content in an inactive window.
  if (!owner_window())
    return;
#if defined(OS_MAC)
  if (!owner_window()->IsActive())
    return;
#else
  if (!owner_window()->widget()->IsActive())
    return;