* 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
PROGRESS_NONE,           
    PROGRESS_INDETERMINATE,  
    PROGRESS_ERROR,          
    PROGRESS_PAUSED,         
    PROGRESS_NORMAL,         
  };
  virtual void SetProgressBar(double progress, const ProgressState state) = 0;
  virtual void SetOverlayIcon(const gfx::Image& overlay,
                              const std::string& description) = 0;
  
  virtual void SetVisibleOnAllWorkspaces(bool visible,
                                         bool visibleOnFullScreen = false) = 0;
  virtual bool IsVisibleOnAllWorkspaces() = 0;
  virtual void SetAutoHideCursor(bool auto_hide);
  
  virtual void SetVibrancy(const std::string& type);
  
  virtual void SetTouchBar(
      const std::vector<mate::PersistentDictionary>& items);