* 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 "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"
#endif
namespace atom {
namespace {
#if defined(OS_WIN)
void FlipWindowStyle(HWND handle, bool on, DWORD flag) {
  DWORD style = ::GetWindowLong(handle, GWL_STYLE);
  if (on)
    style |= flag;
  else
    style &= ~flag;
  ::SetWindowLong(handle, GWL_STYLE, style);
}
#endif
class NativeWindowClientView : public views::ClientView {