### 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

14.2.0 / 15.3.0 / 16.0.0-beta.6

### What operating system are you using?

macOS

### Operating System Version

macOS Catalina 10.15.7

### What arch are you using?

x64

### Last Known Working Electron version

14.0.0 / 15.0.0-beta.2

### Expected Behavior

`<webview>` element should support trasparency. Check the included Fiddle sample.

### Actual Behavior

`<webview>` element transparency is broken

### Testcase Gist URL

_No response_

### Additional Information
[webview-transparency.zip](https://github.com/electron/electron/files/7464785/webview-transparency.zip)

<img width="912" alt="Screen Shot 2021-11-03 at 1 55 47 AM" src="https://user-images.githubusercontent.com/1281234/139971477-2077f353-c17c-417f-9f13-9c5c52139efa.png">
<img width="912" alt="Screen Shot 2021-11-03 at 1 55 53 AM" src="https://user-images.githubusercontent.com/1281234/139971481-49f8afa5-3024-46ba-9339-33c492ba3677.png">

Related to:
https://github.com/electron/electron/blob/2a2a1a834c7072226e9368f36fe0d22a125803f0/shell/browser/api/electron_api_web_contents.cc#L1461

It regressed in this ranges:
- https://github.com/electron/electron/compare/v14.0.0...v14.0.1 (probably #30819)
- https://github.com/electron/electron/compare/v15.0.0-beta.2...v15.0.0-beta.3 (probably #30777)
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,