* 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.h
double aspect_ratio_ = 0.0;
  gfx::Size aspect_ratio_extraSize_;
  
  NativeWindow* parent_ = nullptr;
  
  bool is_modal_ = false;
  
  NativeBrowserView* browser_view_ = nullptr;
  
  base::ObserverList<NativeWindowObserver> observers_;
  base::WeakPtrFactory<NativeWindow> weak_factory_;
  DISALLOW_COPY_AND_ASSIGN(NativeWindow);
};
class NativeWindowRelay
    : public content::WebContentsUserData<NativeWindowRelay> {
 public:
  static const void* const kNativeWindowRelayUserDataKey;
  static void CreateForWebContents(content::WebContents*,
                                   base::WeakPtr<NativeWindow>);
  ~NativeWindowRelay() override;
  NativeWindow* GetNativeWindow() const { return native_window_.get(); }
 private:
  friend class content::WebContentsUserData<NativeWindow>;
  explicit NativeWindowRelay(base::WeakPtr<NativeWindow> window);
  base::WeakPtr<NativeWindow> native_window_;
};
}  
#endif