* 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
UINT_PTR subclass_id,
                                       DWORD_PTR ref_data);
  static LRESULT CALLBACK MouseHookProc(int n_code,
                                        WPARAM w_param,
                                        LPARAM l_param);
#endif
  
  bool ShouldBeEnabled();
  void SetEnabledInternal(bool enabled);
  
  void HandleKeyboardEvent(
      content::WebContents*,
      const content::NativeWebKeyboardEvent& event) override;
  
  ui::WindowShowState GetRestoredState();
  std::unique_ptr<RootView> root_view_;
  
  views::View* focused_view_ = nullptr;
  
  
  
  
  extensions::SizeConstraints old_size_constraints_;
#if defined(USE_X11)
  std::unique_ptr<GlobalMenuBarX11> global_menu_bar_;
  
  std::unique_ptr<WindowStateWatcher> window_state_watcher_;
  
  std::unique_ptr<EventDisabler> event_disabler_;
#endif