### 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
std::string target_id) {
  auto agent_host = content::DevToolsAgentHost::GetForId(target_id);
  WebContents* contents =
      agent_host ? WebContents::From(agent_host->GetWebContents()) : nullptr;
  return contents ? gin::CreateHandle(isolate, contents)
                  : gin::HContents>();
}
std::vector<gin::HContents>> GetAllWebContentsAsV8(
    v8::Isolate* isolate) {
  std::vector<gin::HContents>> list;
  for (auto iter = base::IDMap<WebContents*>::iterator(&GetAllWebContents());
       !iter.IsAtEnd(); iter.Advance()) {
    list.push_back(gin::CreateHandle(isolate, iter.GetCurrentValue()));
  }
  return list;
}
void Initialize(v8::Lobject> exports,
                v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context,
                void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  gin_helper::Dictionary dict(isolate, exports);
  dict.Set("WebContents", WebContents::GetConstructor(context));
  dict.SetMethod("fromId", &WebContentsFromID);
  dict.SetMethod("fromDevToolsTargetId", &WebContentsFromDevToolsTargetID);
  dict.SetMethod("getAllWebContents", &GetAllWebContentsAsV8);
}
e
NODULE_CONTEXT_AWARE(electron_browser_ontents, Initialize)