* 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
params.wm_role_name = "browser-window";
  
  params.wm_class_name = base::ToLowerASCII(name);
  params.wm_class_class = name;
#endif
  widget()->Init(params);
  bool fullscreen = false;
  options.Get(options::kFullscreen, &fullscreen);
  std::string window_type;
  options.Get(options::kType, &window_type);
#if defined(USE_X11)
  
  window_state_watcher_.reset(new WindowStateWatcher(this));
  
  bool use_dark_theme = false;
  if (options.Get(options::kDarkTheme, &use_dark_theme) && use_dark_theme) {
    XDisplay* xdisplay = gfx::GetXDisplay();
    XChangeProperty(xdisplay, GetAcceleratedWidget(),
                    XInternAtom(xdisplay, "_GTK_THEME_VARIANT", x11::False),
                    XInternAtom(xdisplay, "UTF8_STRING", x11::False), 8,
                    PropModeReplace,
                    reinterpret_cast<const unsigned char*>("dark"), 4);
  }
  
  
  std::vector<::Atom> state_atom_list;
  bool skip_taskbar = false;
  if (options.Get(options::kSkipTaskbar, &skip_taskbar) && skip_taskbar) {
    state_atom_list.push_back(GetAtom("_NET_WM_STATE_SKIP_TASKBAR"));
  }