* 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)
  HMENU menu = GetSystemMenu(GetAcceleratedWidget(), false);
  MENUITEMINFO info;
  memset(&info, 0, sizeof(info));
  info.cbSize = sizeof(info);
  info.fMask = MIIM_STATE;
  if (!GetMenuItemInfo(menu, SC_CLOSE, false, &info)) {
    return false;
  }
  return !(info.fState & MFS_DISABLED);
#elif defined(USE_X11)
  return true;
#endif
}
void NativeWindowViews::SetAlwaysOnTop(bool top,
                                       const std::string& level,
                                       int relativeLevel,
                                       std::string* error) {
  if (top != widget()->IsAlwaysOnTop())
    NativeWindow::NotifyWindowAlwaysOnTopChanged();
  widget()->SetAlwaysOnTop(top);
}
bool NativeWindowViews::IsAlwaysOnTop() {
  return widget()->IsAlwaysOnTop();
}
void NativeWindowViews::Center() {