* 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
params.shadow_type = views::Widget::InitParams::SHADOW_TYPE_NONE;
  bool focusable;
  if (options.Get(options::kFocusable, &focusable) && !focusable)
    params.activatable = views::Widget::InitParams::ACTIVATABLE_NO;
#if defined(OS_WIN)
  if (parent)
    params.parent = parent->GetNativeWindow();
  params.native_widget = new AtomDesktopNativeWidgetAura(widget());
  atom_desktop_window_tree_host_win_ = new AtomDesktopWindowTreeHostWin(
      this, widget(),
      static_cast<views::DesktopNativeWidgetAura*>(params.native_widget));
  params.desktop_window_tree_host = atom_desktop_window_tree_host_win_;
#elif defined(USE_X11)
  std::string name = Browser::Get()->GetName();
  
  params.wm_role_name = "browser-window";
  
  params.wm_class_name = base::ToLowerASCII(name);
  params.wm_class_class = name;
#endif
  widget()->Init(params);
  bool fullscreen = false;
  options.Get(options::kFullscreen, &fullscreen);
  std::string window_type;
  options.Get(options::kType, &window_type);
#if defined(USE_X11)
  
  window_state_watcher_.reset(new WindowStateWatcher(this));
  
  bool use_dark_theme = false;