* 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 SetVisibleOnAllWorkspaces(bool visible,
                                 bool visibleOnFullScreen) override;
  bool IsVisibleOnAllWorkspaces() override;
  void SetAutoHideCursor(bool auto_hide) override;
  void SelectPreviousTab() override;
  void SelectNextTab() override;
  void MergeAllWindows() override;
  void MoveTabToNewWindow() override;
  void ToggleTabBar() override;
  bool AddTabbedWindow(NativeWindow* window) override;
  bool SetWindowButtonVisibility(bool visible) override;
  void SetVibrancy(const std::string& type) override;
  void SetTouchBar(
      const std::vector<mate::PersistentDictionary>& items) override;
  void RefreshTouchBarItem(const std::string& item_id) override;
  void SetEscapeTouchBarItem(const mate::PersistentDictionary& item) override;
  gfx::Rect ContentBoundsToWindowBounds(const gfx::Rect& bounds) const override;
  gfx::Rect WindowBoundsToContentBounds(const gfx::Rect& bounds) const override;
  
  void OverrideNSWindowContentView();
  
  void SetStyleMask(bool on, NSUInteger flag);
  void SetCollectionBehavior(bool on, NSUInteger flag);
  enum TitleBarStyle {
    NORMAL,
    HIDDEN,
    HIDDEN_INSET,
    CUSTOM_BUTTONS_ON_HOVER,
  };
  TitleBarStyle title_bar_style() const { return title_bar_style_; }