* 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.cc
root_view_->SetAutoHideMenuBar(auto_hide);
}
bool NativeWindowViews::IsMenuBarAutoHide() {
  return root_view_->IsMenuBarAutoHide();
}
void NativeWindowViews::SetMenuBarVisibility(bool visible) {
  root_view_->SetMenuBarVisibility(visible);
}
bool NativeWindowViews::IsMenuBarVisible() {
  return root_view_->IsMenuBarVisible();
}
void NativeWindowViews::SetVisibleOnAllWorkspaces(bool visible,
                                                  bool visibleOnFullScreen) {
  widget()->SetVisibleOnAllWorkspaces(visible);
}
bool NativeWindowViews::IsVisibleOnAllWorkspaces() {
#if defined(USE_X11)
  
  
  XAtom sticky_atom = GetAtom("_NET_WM_STATE_STICKY");
  std::vector<XAtom> wm_states;
  ui::GetAtomArrayProperty(GetAcceleratedWidget(), "_NET_WM_STATE", &wm_states);
  return std::find(wm_states.begin(), wm_states.end(), sticky_atom) !=
         wm_states.end();
#endif
  return false;
}
gfx::AcceleratedWidget NativeWindowViews::GetAcceleratedWidget() const {