### 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
#include "shell/browser/extensions/electron_extension_ontents_observer.h"
#endif
#if BUILDFLAG(ENABLE_PRINTING)
#include "components/printing/browser/print_manager_utils.h"
#include "printing/backend/print_backend.h"  
#include "printing/mojom/print.mojom.h"
#include "shell/browser/printing/print_preview_message_handler.h"
#include "shell/browser/printing/print_view_manager_electron.h"
#if defined(OS_WIN)
#include "printing/backend/win_helper.h"
#endif
#endif
#if BUILDFLAG(ENABLE_PICTURE_IN_PICTURE)
#include "chrome/browser/picture_in_picture/picture_in_picture_window_manager.h"
#endif
#if BUILDFLAG(ENABLE_PDF_VIEWER)
#include "components/pdf/browser/pdf_ontents_helper.h"  
#include "shell/browser/electron_pdf_ontents_helper_client.h"
#endif
#if BUILDFLAG(ENABLE_PLUGINS)
#include "content/public/browser/plugin_service.h"
#endif
#ifndef MAS_BUILD
#include "chrome/browser/hang_monitor/hang_crash_dump.h"  
#endif
namespace gin {
#if BUILDFLAG(ENABLE_PRINTING)
template <>
struct Converter<printing::mojom::MarginType> {
  static bool FromV8(v8::Isolate* isolate,