* 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
#include "ui/views/controls/webview/webview.h"
#include "ui/views/widget/native_widget_private.h"
#include "ui/views/widget/widget.h"
#include "ui/views/window/client_view.h"
#include "ui/wm/core/shadow_types.h"
#include "ui/wm/core/window_util.h"
#if defined(USE_X11)
#include "atom/browser/browser.h"
#include "atom/browser/ui/views/frameless_view.h"
#include "atom/browser/ui/views/global_menu_bar_x11.h"
#include "atom/browser/ui/views/native_frame_view.h"
#include "atom/browser/ui/x/event_disabler.h"
#include "atom/browser/ui/x/window_state_watcher.h"
#include "atom/browser/ui/x/x_window_utils.h"
#include "base/strings/string_util.h"
#include "chrome/browser/ui/libgtkui/unity_service.h"
#include "ui/base/x/x11_util.h"
#include "ui/gfx/x/x11_types.h"
#include "ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h"
#include "ui/views/window/native_frame_view.h"
#elif defined(OS_WIN)
#include "atom/browser/ui/views/win_frame_view.h"
#include "atom/browser/ui/win/atom_desktop_native_widget_aura.h"
#include "atom/browser/ui/win/atom_desktop_window_tree_host_win.h"
#include "skia/ext/skia_utils_win.h"
#include "ui/base/win/shell.h"
#include "ui/display/display.h"
#include "ui/display/screen.h"
#include "ui/display/win/screen_win.h"
#include "ui/views/widget/desktop_aura/desktop_native_widget_aura.h"