* 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.h
LPARAM l_param);
#endif
  
  bool ShouldBeEnabled();
  void SetEnabledInternal(bool enabled);
  
  void HandleKeyboardEvent(
      content::WebContents*,
      const content::NativeWebKeyboardEvent& event) override;
#if defined(OS_LINUX)
  
  void OnMouseEvent(ui::MouseEvent* event) override;
#endif
  
  ui::WindowShowState GetRestoredState();
  std::unique_ptr<RootView> root_view_;
  
  views::View* focused_view_ = nullptr;
  
  
  
  
  extensions::SizeConstraints old_size_constraints_;
#if defined(USE_X11)
  std::unique_ptr<GlobalMenuBarX11> global_menu_bar_;
  
  std::unique_ptr<WindowStateWatcher> window_state_watcher_;
  
  std::unique_ptr<EventDisabler> event_disabler_;
#endif