### 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
if (IsUnityRunning()) {
    unity::SetDownloadCount(count);
    badge_count_ = count;
    return true;
  } else {
    return false;
  }
}
void Browser::SetLoginItemSettings(LoginItemSettings settings) {}
Browser::LoginItemSettings Browser::GetLoginItemSettings(
    const LoginItemSettings& options) {
  return LoginItemSettings();
}
std::string Browser::GetExecutableFileVersion() const {
  return GetApplicationVersion();
}
std::string Browser::GetExecutableFileProductName() const {
  return GetApplicationName();
}
bool Browser::IsUnityRunning() {
  return unity::IsRunning();
}
bool Browser::IsEmojiPanelSupported() {
  return false;
}
void Browser::ShowAboutPanel() {