* 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
options.Get(options::kHeight, &height);
  gfx::Rect bounds(0, 0, width, height);
  widget_size_ = bounds.size();
  widget()->AddObserver(this);
  views::Widget::InitParams params;
  params.ownership = views::Widget::InitParams::WIDGET_OWNS_NATIVE_WIDGET;
  params.bounds = bounds;
  params.delegate = this;
  params.type = views::Widget::InitParams::TYPE_WINDOW;
  params.remove_standard_frame = !has_frame();
  if (transparent())
    params.opacity = views::Widget::InitParams::TRANSLUCENT_WINDOW;
  
  
  if (transparent() && !has_frame())
    params.shadow_type = views::Widget::InitParams::SHADOW_TYPE_NONE;
  bool focusable;
  if (options.Get(options::kFocusable, &focusable) && !focusable)
    params.activatable = views::Widget::InitParams::ACTIVATABLE_NO;
#if defined(OS_WIN)
  if (parent)
    params.parent = parent->GetNativeWindow();
  params.native_widget = new AtomDesktopNativeWidgetAura(widget());
  atom_desktop_window_tree_host_win_ = new AtomDesktopWindowTreeHostWin(
      this, widget(),
      static_cast<views::DesktopNativeWidgetAura*>(params.native_widget));
  params.desktop_window_tree_host = atom_desktop_window_tree_host_win_;
#elif defined(USE_X11)
  std::string name = Browser::Get()->GetName();