### 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
rwh_impl->disable_hidden_ = !background_throttling_;
  ontents()->GetRenderViewHost()->SetSchedulerThrottling(allowed);
  if (rwh_impl->is_hidden()) {
    rwh_impl->WasShown({});
  }
}
int WebContents::GetProcessID() const {
  return ontents()->GetMainFrame()->GetProcess()->GetID();
}
base::ProcessId WebContents::GetOSProcessID() const {
  base::ProcessHandle process_handle =
      ontents()->GetMainFrame()->GetProcess()->GetProcess().Handle();
  return base::GetProcId(process_handle);
}
WebContents::Type WebContents::GetType() const {
  return type_;
}
bool WebContents::Equal(const WebContents* ontents) const {
  return ID() == ontents->ID();
}
GURL WebContents::GetURL() const {
  return ontents()->GetLastCommittedURL();
}
void WebContents::LoadURL(const GURL& url,
                          const gin_helper::Dictionary& options) {
  if (!url.is_valid() || url.spec().size() > url::kMaxURLChars) {
    Emit("did-fail-load", static_cast<int>(net::ERR_INVALID_URL),
         net::ErrorToShortString(net::ERR_INVALID_URL),
         url.possibly_invalid_spec(), true);
    return;