* 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
bool is_kiosk_ = false;
  bool was_fullscreen_ = false;
  bool zoom_to_page_width_ = false;
  bool fullscreen_window_title_ = false;
  bool resizable_ = true;
  NSInteger attention_request_id_ = 0;  
  
  NSApplicationPresentationOptions kiosk_options_;
  
  TitleBarStyle title_bar_style_ = NORMAL;
  
  
  base::Optional<bool> window_button_visibility_;
  
  bool always_simple_fullscreen_ = false;
  bool is_simple_fullscreen_ = false;
  bool was_maximizable_ = false;
  bool was_movable_ = false;
  NSRect original_frame_;
  NSInteger original_level_;
  NSUInteger simple_fullscreen_mask_;
  base::scoped_nsobject<NSColor> background_color_before_vibrancy_;
  bool transparency_before_vibrancy_ = false;
  
  NSApplicationPresentationOptions simple_fullscreen_options_;
  DISALLOW_COPY_AND_ASSIGN(NativeWindowMac);
};
}  
#endif