* 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
virtual void Invalidate() = 0;
  virtual void SetTitle(const std::string& title) = 0;
  virtual std::string GetTitle() = 0;
  virtual void FlashFrame(bool flash) = 0;
  virtual void SetSkipTaskbar(bool skip) = 0;
  virtual void SetSimpleFullScreen(bool simple_fullscreen) = 0;
  virtual bool IsSimpleFullScreen() = 0;
  virtual void SetKiosk(bool kiosk) = 0;
  virtual bool IsKiosk() = 0;
  virtual void SetBackgroundColor(SkColor color) = 0;
  virtual void SetHasShadow(bool has_shadow) = 0;
  virtual bool HasShadow() = 0;
  virtual void SetOpacity(const double opacity) = 0;
  virtual double GetOpacity() = 0;
  virtual void SetRepresentedFilename(const std::string& filename);
  virtual std::string GetRepresentedFilename();
  virtual void SetDocumentEdited(bool edited);
  virtual bool IsDocumentEdited();
  virtual void SetIgnoreMouseEvents(bool ignore, bool forward) = 0;
  virtual void SetContentProtection(bool enable) = 0;
  virtual void SetFocusable(bool focusable);
  virtual void SetMenu(AtomMenuModel* menu);
  virtual void SetParentWindow(NativeWindow* parent);
  virtual void SetBrowserView(NativeBrowserView* browser_view) = 0;
  virtual gfx::NativeView GetNativeView() const = 0;
  virtual gfx::NativeWindow GetNativeWindow() const = 0;
  virtual gfx::AcceleratedWidget GetAcceleratedWidget() const = 0;
  virtual std::tuple<void*, int> GetNativeWindowHandlePointer() const = 0;
  
  enum ProgressState {