### 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 accept_languages =
      g_browser_process->GetApplicationLocale() + ",";
  for (auto const& language : electron::GetPreferredLanguages()) {
    if (language == g_browser_process->GetApplicationLocale())
      continue;
    accept_languages += language + ",";
  }
  accept_languages.pop_back();
  prefs->accept_languages = accept_languages;
#if defined(OS_LINUX) || defined(OS_WIN)
  
  static const gfx::FontRenderParams params(
      gfx::GetFontRenderParams(gfx::FontRenderParamsQuery(), nullptr));
  prefs->should_antialias_text = params.antialiasing;
  prefs->use_subpixel_positioning = params.subpixel_positioning;
  prefs->hinting = params.hinting;
  prefs->use_autohinter = params.autohinter;
  prefs->use_bitmaps = params.use_bitmaps;
  prefs->subpixel_rendering = params.subpixel_rendering;
#endif
  
  if (auto interval = GetCursorBlinkInterval())
    prefs->caret_blink_interval = *interval;
  
  
  
  if (!WebContentsPreferences::From(ontents())) {
    new WebContentsPreferences(ontents(), options);
  }