* 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
void NotifyWindowWillMove(const gfx::Rect& new_bounds, bool* prevent_default);
  void NotifyWindowMoved();
  void NotifyWindowScrollTouchBegin();
  void NotifyWindowScrollTouchEnd();
  void NotifyWindowSwipe(const std::string& direction);
  void NotifyWindowSheetBegin();
  void NotifyWindowSheetEnd();
  void NotifyWindowEnterFullScreen();
  void NotifyWindowLeaveFullScreen();
  void NotifyWindowEnterHtmlFullScreen();
  void NotifyWindowLeaveHtmlFullScreen();
  void NotifyWindowAlwaysOnTopChanged();
  void NotifyWindowExecuteWindowsCommand(const std::string& command);
  void NotifyTouchBarItemInteraction(const std::string& item_id,
                                     const base::DictionaryValue& details);
  void NotifyNewWindowForTab();
#if defined(OS_WIN)
  void NotifyWindowMessage(UINT message, WPARAM w_param, LPARAM l_param);
#endif
  void AddObserver(NativeWindowObserver* obs) { observers_.AddObserver(obs); }
  void RemoveObserver(NativeWindowObserver* obs) {
    observers_.RemoveObserver(obs);
  }
  views::Widget* widget() const { return widget_.get(); }
  views::View* content_view() const { return content_view_; }
  bool has_frame() const { return has_frame_; }
  void set_has_frame(bool has_frame) { has_frame_ = has_frame; }
  bool transparent() const { return transparent_; }
  bool enable_larger_than_screen() const { return enable_larger_than_screen_; }
  NativeBrowserView* browser_view() const { return browser_view_; }