* 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
#if defined(OS_WIN)
  Microsoft::WRL::ComPtr<ITaskbarList> taskbar;
  if (FAILED(::CoCreateInstance(CLSID_TaskbarList, nullptr,
                                CLSCTX_INPROC_SERVER,
                                IID_PPV_ARGS(&taskbar))) ||
      FAILED(taskbar->HrInit()))
    return;
  if (skip) {
    taskbar->DeleteTab(GetAcceleratedWidget());
  } else {
    taskbar->AddTab(GetAcceleratedWidget());
    taskbar_host_.RestoreThumbarButtons(GetAcceleratedWidget());
  }
#elif defined(USE_X11)
  SetWMSpecState(GetAcceleratedWidget(), skip,
                 GetAtom("_NET_WM_STATE_SKIP_TASKBAR"));
#endif
}
void NativeWindowViews::SetSimpleFullScreen(bool simple_fullscreen) {