* 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
#ifndef ATOM_BROWSER_NATIVE_WINDOW_H_
#define ATOM_BROWSER_NATIVE_WINDOW_H_
#include <map>
#include <memory>
#include <string>
#include <tuple>
#include <vector>
#include "atom/browser/native_window_observer.h"
#include "base/memory/weak_ptr.h"
#include "base/observer_list.h"
#include "base/supports_user_data.h"
#include "content/public/browser/web_contents_user_data.h"
#include "extensions/browser/app_window/size_constraints.h"
#include "ui/views/widget/widget_delegate.h"
class SkRegion;
namespace content {
struct NativeWebKeyboardEvent;
}
namespace gfx {
class Image;