* 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
class Point;
class Rect;
class RectF;
class Size;
}  
namespace mate {
class Dictionary;
class PersistentDictionary;
}  
namespace atom {
class AtomMenuModel;
class NativeBrowserView;
struct DraggableRegion;
class NativeWindow : public base::SupportsUserData,
                     public views::WidgetDelegate {
 public:
  ~NativeWindow() override;
  
  
  static NativeWindow* Create(const mate::Dictionary& options,
                              NativeWindow* parent = nullptr);
  void InitFromOptions(const mate::Dictionary& options);
  virtual void SetContentView(views::View* view) = 0;
  virtual void Close() = 0;
  virtual void CloseImmediately() = 0;
  virtual bool IsClosed() const;
  virtual void Focus(bool focus) = 0;
  virtual bool IsFocused() = 0;
  virtual void Show() = 0;
  virtual void ShowInactive() = 0;