* 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
return weak_factory_.GetWeakPtr();
  }
  
  virtual void HandleKeyboardEvent(
      content::WebContents*,
      const content::NativeWebKeyboardEvent& event) {}
  
  
  void NotifyWindowRequestPreferredWith(int* width);
  void NotifyWindowCloseButtonClicked();
  void NotifyWindowClosed();
  void NotifyWindowEndSession();
  void NotifyWindowBlur();
  void NotifyWindowFocus();
  void NotifyWindowShow();
  void NotifyWindowHide();
  void NotifyWindowMaximize();
  void NotifyWindowUnmaximize();
  void NotifyWindowMinimize();
  void NotifyWindowRestore();
  void NotifyWindowMove();
  void NotifyWindowWillResize(const gfx::Rect& new_bounds,
                              bool* prevent_default);
  void NotifyWindowResize();