* 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.h
double aspect_ratio_ = 0.0;
  gfx::Size aspect_ratio_extraSize_;
  
  NativeWindow* parent_ = nullptr;
  
  bool is_modal_ = false;
  
  NativeBrowserView* browser_view_ = nullptr;
  
  base::ObserverList<NativeWindowObserver> observers_;
  base::WeakPtrFactory<NativeWindow> weak_factory_;
  DISALLOW_COPY_AND_ASSIGN(NativeWindow);
};
class NativeWindowRelay
    : public content::WebContentsUserData<NativeWindowRelay> {
 public:
  static const void* const kNativeWindowRelayUserDataKey;
  static void CreateForWebContents(content::WebContents*,
                                   base::WeakPtr<NativeWindow>);
  ~NativeWindowRelay() override;
  NativeWindow* GetNativeWindow() const { return native_window_.get(); }
 private:
  friend class content::WebContentsUserData<NativeWindow>;
  explicit NativeWindowRelay(base::WeakPtr<NativeWindow> window);
  base::WeakPtr<NativeWindow> native_window_;
};
}  
#endif