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### Preflight Checklist
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* [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 for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:**
  * v8.0.0-beta.2
* **Operating System:**
  * macOS 10.14.5
* **Last Known Working Electron version:**
  * v6.1.X 

### Expected Behavior
No warnings

### Actual Behavior
The warning

```
[28455:1113/114404.211707:ERROR:CONSOLE(3361)] "Uncaught TypeError: sender.emit is not iterable (cannot read property Symbol(Symbol.iterator))", source: electron/js2c/sandbox_bundle.js (3361)
```

is printed in the console constantly.

### To Reproduce
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```sh
$ git clone $YOUR_URL -b $BRANCH
$ npm install
$ npm start || electron .
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### Screenshots

![image](https://user-images.githubusercontent.com/380914/68798485-5cd39b00-060b-11ea-80d9-1f62cbb97e52.png)


shell/browser/api/atom_api_web_contents.cc
const GURL& target_url,
    const content::Referrer& referrer,
    const std::string& frame_name,
    WindowOpenDisposition disposition,
    const std::vector<std::string>& features,
    const scoped_refptr<network::ResourceRequestBody>& body) {
  if (type_ == Type::BROWSER_WINDOW || type_ == Type::OFF_SCREEN)
    Emit("-new-window", target_url, frame_name, disposition, features, body,
         referrer);
  else
    Emit("new-window", target_url, frame_name, disposition, features);
}
void Ws::WsCreated(content::Ws* source_contents,
                                     int opener_render_process_id,
                                     int opener_render_frame_id,
                                     const std::string& frame_name,
                                     const GURL& target_url,
                                     content::Ws* new_contents) {
  ChildWsTracker::CreateForWs(new_contents);
  auto* tracker = ChildWsTracker::FromWs(new_contents);
  tracker->url = target_url;
  tracker->frame_name = frame_name;
}
void Ws::AddNewContents(