* 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
void SetOverlayIcon(const gfx::Image& overlay,
                      const std::string& description) override;
  void SetProgressBar(double progress, const ProgressState state) override;
  void SetAutoHideMenuBar(bool auto_hide) override;
  bool IsMenuBarAutoHide() override;
  void SetMenuBarVisibility(bool visible) override;
  bool IsMenuBarVisible() override;
  void SetVisibleOnAllWorkspaces(bool visible,
                                 bool visibleOnFullScreen) override;
  bool IsVisibleOnAllWorkspaces() override;
  gfx::AcceleratedWidget GetAcceleratedWidget() const override;
  gfx::Rect ContentBoundsToWindowBounds(const gfx::Rect& bounds) const override;
  gfx::Rect WindowBoundsToContentBounds(const gfx::Rect& bounds) const override;
  void UpdateDraggableRegions(std::unique_ptr<SkRegion> region);
  void IncrementChildModals();
  void DecrementChildModals();
#if defined(OS_WIN)
  void SetIcon(HICON small_icon, HICON app_icon);
#elif defined(USE_X11)
  void SetIcon(const gfx::ImageSkia& icon);
#endif
  SkRegion* draggable_region() const { return draggable_region_.get(); }
#if defined(OS_WIN)
  TaskbarHost& taskbar_host() { return taskbar_host_; }
#endif
 private:
  
  void OnWidgetActivationChanged(views::Widget* widget, bool active) override;
  void OnWidgetBoundsChanged(views::Widget* widget,
                             const gfx::Rect& bounds) override;