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

### Issue Details

It would appear that `app.focus()` stopped working in Electron 6, and remains broken in Electron 8.

* **Electron Version:**
  * 8.0.3
* **Operating System:**
  * macOS
* **Last Known Working Electron version:**
  * 5.0.13

### Expected Behavior
`app.focus()` should bring the application to the foreground.

### Actual Behavior
`app.focus()` does nothing.

### To Reproduce

Fiddle: https://gist.github.com/a772265b312329e3f3bc1feb7b6aff7d

1) Start this fiddle.
2) As soon as the window pops up, select another application so that Electron is in the background.
3) Within 5 seconds, the app will attempt to focus itself to no avail.

shell/browser/browser_linux.cc
for (auto* const window : WindowList::GetWindows()) {
    if (window->IsVisible()) {
      window->Focus(true);
      break;
    }
  }
}
void Browser::AddRecentDocument(const base::FilePath& path) {}
void Browser::ClearRecentDocuments() {}
void Browser::SetAppUserModelID(const base::string16& name) {}
bool Browser::SetAsDefaultProtocolClient(const std::string& protocol,
                                         gin_helper::Arguments* args) {
  return SetDefaultWebClient(protocol);
}
bool Browser::IsDefaultProtocolClient(const std::string& protocol,