* 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
SetFullScreen(simple_fullscreen);
}
bool NativeWindowViews::IsSimpleFullScreen() {
  return IsFullscreen();
}
void NativeWindowViews::SetKiosk(bool kiosk) {
  SetFullScreen(kiosk);
}
bool NativeWindowViews::IsKiosk() {
  return IsFullscreen();
}
void NativeWindowViews::SetBackgroundColor(SkColor background_color) {
  
  root_view_->SetBackground(views::CreateSolidBackground(background_color));
#if defined(OS_WIN)
  
  HBRUSH brush = CreateSolidBrush(skia::SkColorToCOLORREF(background_color));
  ULONG_PTR previous_brush =
      SetClassLongPtr(GetAcceleratedWidget(), GCLP_HBRBACKGROUND,
                      reinterpret_cast<LONG_PTR>(brush));
  if (previous_brush)
    DeleteObject((HBRUSH)previous_brush);
  InvalidateRect(GetAcceleratedWidget(), NULL, 1);
#endif
}
void NativeWindowViews::SetHasShadow(bool has_shadow) {