### Preflight Checklist

- [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/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

14.0.0-beta.12

### What operating system are you using?

Windows

### Operating System Version

10.0.19042 Build 19042

### What arch are you using?

x64

### Last Known Working Electron version

13

### Expected Behavior

Window should be transparent

### Actual Behavior

Open a window with transparency enabled and delete the dom, the window will be white instead of transparent.
**Also seems like settings the backgroundColor property has no longer an effect.** I always see a white window

```
const {app, BrowserWindow} = require('electron')
const path = require("path");

function createWindow () {
	const mainWindow = new BrowserWindow({
		transparent: true,
		frame: false,
	})

	mainWindow.loadURL('https://example.com')
	mainWindow.webContents.openDevTools()
}

app.whenReady().then(createWindow)
```

### Testcase Gist URL

https://gist.github.com/e124340107f6c4a3ade8b36d98a16b9a

### Additional Information

_No response_
shell/browser/api/electron_api_web_contents.cc
const std::string& content,
                                     bool save_as) {
  base::FilePath path;
  auto it = saved_files_.find(url);
  if (it != saved_files_.end() && !save_as) {
    path = it->second;
  } else {
    file_dialog::DialogSettings settings;
    settings.parent_window = owner_window();
    settings.force_detached = offscreen_;
    settings.title = url;
    settings.default_path = base::FilePath::FromUTF8Unsafe(url);
    if (!file_dialog::ShowSaveDialogSync(settings, &path)) {
      base::Value url_value(url);
      inspectable_ontents_->CallClientFunction(
          "DevToolsAPI.canceledSaveURL", &url_value, nullptr, nullptr);
      return;
    }
  }
  saved_files_[url] = path;
   Notify DevTools.
  base::Value url_value(url);
  base::Value file_system_path_value(path.AsUTF8Unsafe());
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.savedURL", &url_value, &file_system_path_value, nullptr);
  file_task_runner_->PostTask(FROM_HERE,
                              base::BindOnce(&WriteToFile, path, content));
}
void WebContents::DevToolsAppendToFile(const std::string& url,