* 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 (fullscreen)
    last_window_state_ = ui::SHOW_STATE_FULLSCREEN;
  else
    last_window_state_ = ui::SHOW_STATE_NORMAL;
  last_normal_bounds_ = GetBounds();
#endif
}
NativeWindowViews::~NativeWindowViews() {
  widget()->RemoveObserver(this);
#if defined(OS_WIN)
  
  SetForwardMouseMessages(false);
#endif
}
void NativeWindowViews::SetContentView(views::View* view) {
  if (content_view()) {
    root_view_->RemoveChildView(content_view());
    if (browser_view()) {
      content_view()->RemoveChildView(
          browser_view()->GetInspectableWebContentsView()->GetView());
      set_browser_view(nullptr);
    }
  }
  set_content_view(view);
  focused_view_ = view;
  root_view_->AddChildView(content_view());
  root_view_->Layout();
}
void NativeWindowViews::Close() {