### 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 "third_party/blink/public/common/input/web_input_event.h"
#include "third_party/blink/public/common/messaging/transferable_message_mojom_traits.h"
#include "third_party/blink/public/common/page/page_zoom.h"
#include "third_party/blink/public/mojom/frame/find_in_page.mojom.h"
#include "third_party/blink/public/mojom/frame/fullscreen.mojom.h"
#include "third_party/blink/public/mojom/messaging/transferable_message.mojom.h"
#include "third_party/blink/public/mojom/renderer_preferences.mojom.h"
#include "ui/base/cursor/cursor.h"
#include "ui/base/cursor/mojom/cursor_type.mojom-shared.h"
#include "ui/display/screen.h"
#include "ui/events/base_event_utils.h"
#if BUILDFLAG(ENABLE_OSR)
#include "shell/browser/osr/osr_render_widget_host_view.h"
#include "shell/browser/osr/osr_ontents_view.h"
#endif
#if !defined(OS_MAC)
#include "ui/aura/window.h"
#else
#include "ui/base/cocoa/defaults_utils.h"
#endif
#if defined(OS_LINUX)
#include "ui/views/linux_ui/linux_ui.h"
#endif
#if defined(OS_LINUX) || defined(OS_WIN)
#include "ui/gfx/font_render_params.h"
#endif
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
#include "extensions/browser/script_executor.h"
#include "extensions/browser/view_type_utils.h"
#include "extensions/common/mojom/view_type.mojom.h"