### 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
content::RenderFrameHost* frame,
    content::EyeDropperListener* listener) {
  return ShowEyeDropper(frame, listener);
}
void WebContents::RunFileChooser(
    content::RenderFrameHost* render_frame_host,
    scoped_refptr<content::FileSelectListener> listener,
    const blink::mojom::FileChooserParams& params) {
  FileSelectHelper::RunFileChooser(render_frame_host, std::move(listener),
                                   params);
}
void WebContents::EnumerateDirectory(
    content::WebContents* ontents,
    scoped_refptr<content::FileSelectListener> listener,
    const base::FilePath& path) {
  FileSelectHelper::EnumerateDirectory(ontents, std::move(listener), path);
}
bool WebContents::IsFullscreenForTabOrPending(
    const content::WebContents* source) {
  return html_fullscreen_;
}
blink::SecurityStyle WebContents::GetSecurityStyle(
    content::WebContents* ontents,
    content::SecurityStyleExplanations* security_style_explanations) {
  auto state = security_state::GetVisibleSecurityState(ontents);
  auto security_level = security_state::GetSecurityLevel(*state, false);
  return security_state::GetSecurityStyle(security_level, *state,
                                          security_style_explanations);
}
bool WebContents::TakeFocus(content::WebContents* source, bool reverse) {