### 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
const std::string& channel,
                          blink::CloneableMessage arguments,
                          content::RenderFrameHost* render_frame_host) {
  TRACE_EVENT1("electron", "WebContents::Message", "channel", channel);
   webContents.emit('-ipc-message', new Event(), internal, channel,
   arguments);
  EmitWithSender("-ipc-message", render_frame_host,
                 electron::mojom::ElectronBrowser::InvokeCallback(), internal,
                 channel, std::move(arguments));
}
void WebContents::Invoke(
    bool internal,
    const std::string& channel,
    blink::CloneableMessage arguments,
    electron::mojom::ElectronBrowser::InvokeCallback callback,
    content::RenderFrameHost* render_frame_host) {
  TRACE_EVENT1("electron", "WebContents::Invoke", "channel", channel);
   webContents.emit('-ipc-invoke', new Event(), internal, channel, arguments);
  EmitWithSender("-ipc-invoke", render_frame_host, std::move(callback),
                 internal, channel, std::move(arguments));
}
void WebContents::OnFirstNonEmptyLayout(
    content::RenderFrameHost* render_frame_host) {
  if (render_frame_host == ontents()->GetMainFrame()) {
    Emit("ready-to-show");
  }
}
void WebContents::ReceivePostMessage(