* 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_views.h
void SetMenuBarVisibility(bool visible) override;
  bool IsMenuBarVisible() override;
  void SetVisibleOnAllWorkspaces(bool visible,
                                 bool visibleOnFullScreen) override;
  bool IsVisibleOnAllWorkspaces() override;
  gfx::AcceleratedWidget GetAcceleratedWidget() const override;
  std::tuple<void*, int> GetNativeWindowHandlePointer() const override;
  gfx::Rect ContentBoundsToWindowBounds(const gfx::Rect& bounds) const override;
  gfx::Rect WindowBoundsToContentBounds(const gfx::Rect& bounds) const override;
  void UpdateDraggableRegions(std::unique_ptr<SkRegion> region);
  void IncrementChildModals();
  void DecrementChildModals();
#if defined(OS_WIN)
  void SetIcon(HICON small_icon, HICON app_icon);
#elif defined(USE_X11)
  void SetIcon(const gfx::ImageSkia& icon);
#endif
  SkRegion* draggable_region() const { return draggable_region_.get(); }
#if defined(OS_WIN)
  TaskbarHost& taskbar_host() { return taskbar_host_; }
#endif
 private:
  
  void OnWidgetActivationChanged(views::Widget* widget, bool active) override;
  void OnWidgetBoundsChanged(views::Widget* widget,
                             const gfx::Rect& bounds) override;
  
  void DeleteDelegate() override;
  views::View* GetInitiallyFocusedView() override;
  bool CanResize() const override;