* 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 Center() = 0;
  virtual void Invalidate() = 0;
  virtual void SetTitle(const std::string& title) = 0;
  virtual std::string GetTitle() = 0;
  virtual void FlashFrame(bool flash) = 0;
  virtual void SetSkipTaskbar(bool skip) = 0;
  virtual void SetSimpleFullScreen(bool simple_fullscreen) = 0;
  virtual bool IsSimpleFullScreen() = 0;
  virtual void SetKiosk(bool kiosk) = 0;
  virtual bool IsKiosk() = 0;
  virtual void SetBackgroundColor(SkColor color) = 0;
  virtual void SetHasShadow(bool has_shadow) = 0;
  virtual bool HasShadow() = 0;
  virtual void SetOpacity(const double opacity) = 0;
  virtual double GetOpacity() = 0;
  virtual void SetRepresentedFilename(const std::string& filename);
  virtual std::string GetRepresentedFilename();
  virtual void SetDocumentEdited(bool edited);
  virtual bool IsDocumentEdited();
  virtual void SetIgnoreMouseEvents(bool ignore, bool forward) = 0;
  virtual void SetContentProtection(bool enable) = 0;
  virtual void SetFocusable(bool focusable);
  virtual void SetMenu(AtomMenuModel* menu);
  virtual void SetParentWindow(NativeWindow* parent);
  virtual void SetBrowserView(NativeBrowserView* browser_view) = 0;
  virtual gfx::NativeView GetNativeView() const = 0;
  virtual gfx::NativeWindow GetNativeWindow() const = 0;
  virtual gfx::AcceleratedWidget GetAcceleratedWidget() const = 0;
  
  enum ProgressState {