* 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_mac.h
AtomPreviewItem* preview_item() const { return preview_item_.get(); }
  AtomTouchBar* touch_bar() const { return touch_bar_.get(); }
  bool zoom_to_page_width() const { return zoom_to_page_width_; }
  bool fullscreen_window_title() const { return fullscreen_window_title_; }
  bool simple_fullscreen() const { return always_simple_fullscreen_; }
 protected:
  
  bool CanResize() const override;
  views::View* GetContentsView() override;
 private:
  
  void AddContentViewLayers();
  void InternalSetParentWindow(NativeWindow* parent, bool attach);
  void ShowWindowButton(NSWindowButton button);
  void SetForwardMouseMessages(bool forward);
  AtomNSWindow* window_;  
  base::scoped_nsobject<AtomNSWindowDelegate> window_delegate_;
  base::scoped_nsobject<AtomPreviewItem> preview_item_;
  base::scoped_nsobject<AtomTouchBar> touch_bar_;
  base::scoped_nsobject<CustomWindowButtonView> buttons_view_;
  
  id wheel_event_monitor_;
  
  base::scoped_nsobject<FullSizeContentView> container_view_;
  
  std::unique_ptr<RootViewMac> root_view_;