* 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
virtual void RefreshTouchBarItem(const std::string& item_id);
  virtual void SetEscapeTouchBarItem(const mate::PersistentDictionary& item);
  
  virtual void SelectPreviousTab();
  virtual void SelectNextTab();
  virtual void MergeAllWindows();
  virtual void MoveTabToNewWindow();
  virtual void ToggleTabBar();
  virtual bool AddTabbedWindow(NativeWindow* window);
  
  virtual bool SetWindowButtonVisibility(bool visible);
  
  virtual void SetAutoHideMenuBar(bool auto_hide);
  virtual bool IsMenuBarAutoHide();
  virtual void SetMenuBarVisibility(bool visible);
  virtual bool IsMenuBarVisible();
  
  double GetAspectRatio();
  gfx::Size GetAspectRatioExtraSize();
  virtual void SetAspectRatio(double aspect_ratio, const gfx::Size& extra_size);
  
  virtual void PreviewFile(const std::string& path,
                           const std::string& display_name);
  virtual void CloseFilePreview();
  
  virtual gfx::Rect ContentBoundsToWindowBounds(
      const gfx::Rect& bounds) const = 0;
  virtual gfx::Rect WindowBoundsToContentBounds(
      const gfx::Rect& bounds) const = 0;
  base::WeakPtr<NativeWindow> GetWeakPtr() {