* 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_views.h
static std::set<NativeWindowViews*> forwarding_windows_;
  static HHOOK mouse_hook_;
  bool forwarding_mouse_messages_ = false;
  HWND legacy_window_ = NULL;
  bool layered_ = false;
#endif
  
  std::unique_ptr<views::UnhandledKeyboardEventHandler> keyboard_event_handler_;
  
  
  std::unique_ptr<SkRegion> draggable_region_;  
  
  bool is_enabled_ = true;
  
  
  unsigned int num_modal_children_ = 0;
  bool use_content_size_ = false;
  bool movable_ = true;
  bool resizable_ = true;
  bool maximizable_ = true;
  bool minimizable_ = true;
  bool fullscreenable_ = true;
  std::string title_;
  gfx::Size widget_size_;
  double opacity_ = 1.0;
  DISALLOW_COPY_AND_ASSIGN(NativeWindowViews);
};
}  
#endif