* 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
void NotifyWindowWillMove(const gfx::Rect& new_bounds, bool* prevent_default);
  void NotifyWindowMoved();
  void NotifyWindowScrollTouchBegin();
  void NotifyWindowScrollTouchEnd();
  void NotifyWindowSwipe(const std::string& direction);
  void NotifyWindowSheetBegin();
  void NotifyWindowSheetEnd();
  void NotifyWindowEnterFullScreen();
  void NotifyWindowLeaveFullScreen();
  void NotifyWindowEnterHtmlFullScreen();
  void NotifyWindowLeaveHtmlFullScreen();
  void NotifyWindowAlwaysOnTopChanged();
  void NotifyWindowExecuteAppCommand(const std::string& command);
  void NotifyTouchBarItemInteraction(const std::string& item_id,
                                     const base::DictionaryValue& details);
  void NotifyNewWindowForTab();
#if defined(OS_WIN)
  void NotifyWindowMessage(UINT message, WPARAM w_param, LPARAM l_param);
#endif
  void AddObserver(NativeWindowObserver* obs) { observers_.AddObserver(obs); }
  void RemoveObserver(NativeWindowObserver* obs) {
    observers_.RemoveObserver(obs);
  }
  views::Widget* widget() const { return widget_.get(); }
  views::View* content_view() const { return content_view_; }
  bool has_frame() const { return has_frame_; }
  void set_has_frame(bool has_frame) { has_frame_ = has_frame; }
  bool transparent() const { return transparent_; }
  bool enable_larger_than_screen() const { return enable_larger_than_screen_; }
  NativeBrowserView* browser_view() const { return browser_view_; }