* 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
static std::set<NativeWindowViews*> forwarding_windows_;
  static HHOOK mouse_hook_;
  bool forwarding_mouse_messages_ = false;
  HWND legacy_window_ = NULL;
  bool layered_ = false;
#endif
  
  std::unique_ptr<views::UnhandledKeyboardEventHandler> keyboard_event_handler_;
  
  
  std::unique_ptr<SkRegion> draggable_region_;  
  
  bool is_enabled_ = true;
  
  
  unsigned int num_modal_children_ = 0;
  bool use_content_size_ = false;
  bool movable_ = true;
  bool resizable_ = true;
  bool maximizable_ = true;
  bool minimizable_ = true;
  bool fullscreenable_ = true;
  std::string title_;
  gfx::Size widget_size_;
  double opacity_ = 1.0;
  DISALLOW_COPY_AND_ASSIGN(NativeWindowViews);
};
}  
#endif