* 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 RefreshTouchBarItem(const std::string& item_id);
  virtual void SetEscapeTouchBarItem(const mate::PersistentDictionary& item);
  
  virtual void SelectPreviousTab();
  virtual void SelectNextTab();
  virtual void MergeAllWindows();
  virtual void MoveTabToNewWindow();
  virtual void ToggleTabBar();
  virtual bool AddTabbedWindow(NativeWindow* window);
  
  virtual bool SetWindowButtonVisibility(bool visible);
  
  virtual void SetAutoHideMenuBar(bool auto_hide);
  virtual bool IsMenuBarAutoHide();
  virtual void SetMenuBarVisibility(bool visible);
  virtual bool IsMenuBarVisible();
  
  double GetAspectRatio();
  gfx::Size GetAspectRatioExtraSize();
  virtual void SetAspectRatio(double aspect_ratio, const gfx::Size& extra_size);
  
  virtual void PreviewFile(const std::string& path,
                           const std::string& display_name);
  virtual void CloseFilePreview();
  
  virtual gfx::Rect ContentBoundsToWindowBounds(
      const gfx::Rect& bounds) const = 0;
  virtual gfx::Rect WindowBoundsToContentBounds(
      const gfx::Rect& bounds) const = 0;
  base::WeakPtr<NativeWindow> GetWeakPtr() {