* 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/native_window_mac.h
AtomPreviewItem* preview_item() const { return preview_item_.get(); }
  AtomTouchBar* touch_bar() const { return touch_bar_.get(); }
  bool zoom_to_page_width() const { return zoom_to_page_width_; }
  bool fullscreen_window_title() const { return fullscreen_window_title_; }
  bool simple_fullscreen() const { return always_simple_fullscreen_; }
 protected:
  
  bool CanResize() const override;
  views::View* GetContentsView() override;
 private:
  
  void AddContentViewLayers();
  void InternalSetParentWindow(NativeWindow* parent, bool attach);
  void ShowWindowButton(NSWindowButton button);
  void SetForwardMouseMessages(bool forward);
  AtomNSWindow* window_;  
  base::scoped_nsobject<AtomNSWindowDelegate> window_delegate_;
  base::scoped_nsobject<AtomPreviewItem> preview_item_;
  base::scoped_nsobject<AtomTouchBar> touch_bar_;
  base::scoped_nsobject<CustomWindowButtonView> buttons_view_;
  
  id wheel_event_monitor_;
  
  base::scoped_nsobject<FullSizeContentView> container_view_;
  
  std::unique_ptr<RootViewMac> root_view_;