* 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
window_icon_ = base::win::ScopedHICON(CopyIcon(window_icon));
  app_icon_ = base::win::ScopedHICON(CopyIcon(app_icon));
  HWND hwnd = GetAcceleratedWidget();
  SendMessage(hwnd, WM_SETICON, ICON_SMALL,
              reinterpret_cast<LPARAM>(window_icon_.get()));
  SendMessage(hwnd, WM_SETICON, ICON_BIG,
              reinterpret_cast<LPARAM>(app_icon_.get()));
}
#elif defined(USE_X11)
void NativeWindowViews::SetIcon(const gfx::ImageSkia& icon) {
  auto* tree_host = static_cast<views::DesktopWindowTreeHost*>(
      views::DesktopWindowTreeHostX11::GetHostForXID(GetAcceleratedWidget()));
  tree_host->SetWindowIcons(icon, icon);
}
#endif
void NativeWindowViews::OnWidgetActivationChanged(views::Widget* changed_widget,
                                                  bool active) {
  if (changed_widget != widget())
    return;
  if (active)
    NativeWindow::NotifyWindowFocus();
  else
    NativeWindow::NotifyWindowBlur();
  
  if (!active && IsMenuBarAutoHide() && IsMenuBarVisible())
    SetMenuBarVisibility(false);
  root_view_->ResetAltState();
}
void NativeWindowViews::OnWidgetBoundsChanged(views::Widget* changed_widget,