### 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
.SetMethod("setFrameRate", &WebContents::SetFrameRate)
      .SetMethod("getFrameRate", &WebContents::GetFrameRate)
#endif
      .SetMethod("invalidate", &WebContents::Invalidate)
      .SetMethod("setZoomLevel", &WebContents::SetZoomLevel)
      .SetMethod("getZoomLevel", &WebContents::GetZoomLevel)
      .SetMethod("setZoomFactor", &WebContents::SetZoomFactor)
      .SetMethod("getZoomFactor", &WebContents::GetZoomFactor)
      .SetMethod("getType", &WebContents::GetType)
      .SetMethod("_getPreloadPaths", &WebContents::GetPreloadPaths)
      .SetMethod("getLastWebPreferences", &WebContents::GetLastWebPreferences)
      .SetMethod("getOwnerBrowserWindow", &WebContents::GetOwnerBrowserWindow)
      .SetMethod("inspectServiceWorker", &WebContents::InspectServiceWorker)
      .SetMethod("inspectSharedWorker", &WebContents::InspectSharedWorker)
      .SetMethod("inspectSharedWorkerById",
                 &WebContents::InspectSharedWorkerById)
      .SetMethod("getAllSharedWorkers", &WebContents::GetAllSharedWorkers)
#if BUILDFLAG(ENABLE_PRINTING)
      .SetMethod("_print", &WebContents::Print)
      .SetMethod("_printToPDF", &WebContents::PrintToPDF)
#endif
      .SetMethod("_setNextChildWebPreferences",
                 &WebContents::SetNextChildWebPreferences)
      .SetMethod("addWorkSpace", &WebContents::AddWorkSpace)
      .SetMethod("removeWorkSpace", &WebContents::RemoveWorkSpace)
      .SetMethod("showDefinitionForSelection",
                 &WebContents::ShowDefinitionForSelection)
      .SetMethod("copyImageAt", &WebContents::CopyImageAt)
      .SetMethod("capturePage", &WebContents::CapturePage)
      .SetMethod("setEmbedder", &WebContents::SetEmbedder)