* 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_mac.h
void Center() override;
  void Invalidate() override;
  void SetTitle(const std::string& title) override;
  std::string GetTitle() override;
  void FlashFrame(bool flash) override;
  void SetSkipTaskbar(bool skip) override;
  void SetSimpleFullScreen(bool simple_fullscreen) override;
  bool IsSimpleFullScreen() override;
  void SetKiosk(bool kiosk) override;
  bool IsKiosk() override;
  void SetBackgroundColor(SkColor color) override;
  void SetHasShadow(bool has_shadow) override;
  bool HasShadow() override;
  void SetOpacity(const double opacity) override;
  double GetOpacity() override;
  void SetRepresentedFilename(const std::string& filename) override;
  std::string GetRepresentedFilename() override;
  void SetDocumentEdited(bool edited) override;
  bool IsDocumentEdited() override;
  void SetIgnoreMouseEvents(bool ignore, bool forward) override;
  void SetContentProtection(bool enable) override;
  void SetBrowserView(NativeBrowserView* browser_view) override;
  void SetParentWindow(NativeWindow* parent) override;
  gfx::NativeView GetNativeView() const override;
  gfx::NativeWindow GetNativeWindow() const override;
  gfx::AcceleratedWidget GetAcceleratedWidget() const override;
  std::tuple<void*, int> GetNativeWindowHandlePointer() const override;
  void SetProgressBar(double progress, const ProgressState state) override;
  void SetOverlayIcon(const gfx::Image& overlay,
                      const std::string& description) override;