* Output of `node_modules/.bin/electron --version`: 4.0.0-beta.8
* Operating System (Platform and Version): Windows 10 1803
* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable): 4.0.0-beta.7

**Expected Behavior**
`BrowserWindow.getNativeWindowHandle()` should return different value for different window

**Actual behavior**
`BrowserWindow.getNativeWindowHandle()` returns same value for different window, which is not true nativeWindowHandle for any of them.

**To Reproduce**
Open two `BrowserWindow`, and run `getNativeWindowHandle` on both `BrowserWindow`
```js
const { remote } = require('electron')
w1 = new remote.BrowserWindow()
w2 = new remote.BrowserWindow()
console.log(w1.getNativeWindowHandle(), w2.getNativeWindowHandle())
```
![image](https://user-images.githubusercontent.com/8194131/49328509-7411a800-f5b5-11e8-9a24-827aa589d937.png)


atom/browser/native_window_views.cc
#include "atom/browser/ui/views/root_view.h"
#include "atom/browser/web_contents_preferences.h"
#include "atom/browser/web_view_manager.h"
#include "atom/browser/window_list.h"
#include "atom/common/atom_constants.h"
#include "atom/common/draggable_region.h"
#include "atom/common/native_mate_converters/image_converter.h"
#include "atom/common/options_switches.h"
#include "base/strings/utf_string_conversions.h"
#include "content/public/browser/browser_thread.h"
#include "native_mate/dictionary.h"
#include "ui/aura/window_tree_host.h"
#include "ui/base/hit_test.h"
#include "ui/gfx/image/image.h"
#include "ui/views/background.h"
#include "ui/views/controls/webview/unhandled_keyboard_event_handler.h"
#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"