* Output of `node_modules/.bin/electron --version`:
v3.0.6
v4.0.0-beta.5

* Operating System (Platform and Version):
macOS High Siearra 10.13.6

* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable):
v2.0.11

**Expected Behavior**
BrowserWindow.getNativeWindowHandle() must return non-nullable result.

**Actual behavior**
BrowserWindow.getNativeWindowHandle() returns null.

**To Reproduce**

***Sample code***
```
const { app, BrowserWindow } = require('electron');

let win

app
	.on('ready', () => {
		win = new BrowserWindow({width: 800, height: 600})
		win.loadURL('https://www.google.com')
		
		win.on('closed', () => win = null);

		const handle = win.getNativeWindowHandle();
		console.log('Current window: ' + handle.toString('hex'));
	})
	.on('window-all-closed', () => app.quit());
```

***Expected output***
Something different than `Current window: 0000000000000000`

***Actual output***
`Current window: 0000000000000000`
atom/browser/api/atom_api_top_level_window.cc
const std::string& description) {
  window_->SetOverlayIcon(overlay, description);
}
void TopLevelWindow::SetVisibleOnAllWorkspaces(bool visible,
                                               mate::Arguments* args) {
  mate::Dictionary options;
  bool visibleOnFullScreen = false;
  args->GetNext(&options) &&
      options.Get("visibleOnFullScreen", &visibleOnFullScreen);
  return window_->SetVisibleOnAllWorkspaces(visible, visibleOnFullScreen);
}
bool TopLevelWindow::IsVisibleOnAllWorkspaces() {
  return window_->IsVisibleOnAllWorkspaces();
}
void TopLevelWindow::SetAutoHideCursor(bool auto_hide) {
  window_->SetAutoHideCursor(auto_hide);
}
void TopLevelWindow::SetVibrancy(v8::Isolate* isolate,
                                 v8::Local<v8::Value> value) {
  std::string type = mate::V8ToString(value);
  window_->SetVibrancy(type);
}
void TopLevelWindow::SetTouchBar(
    const std::vector<mate::PersistentDictionary>& items) {
  window_->SetTouchBar(items);
}
void TopLevelWindow::RefreshTouchBarItem(const std::string& item_id) {
  window_->RefreshTouchBarItem(item_id);
}
void TopLevelWindow::SetEscapeTouchBarItem(