* 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.cc
public:
  NativeWindowClientView(views::Widget* widget,
                         views::View* root_view,
                         NativeWindowViews* window)
      : views::ClientView(widget, root_view), window_(window) {}
  ~NativeWindowClientView() override = default;
  bool CanClose() override {
    window_->NotifyWindowCloseButtonClicked();
    return false;
  }
 private:
  NativeWindowViews* window_;
  DISALLOW_COPY_AND_ASSIGN(NativeWindowClientView);
};
}  
NativeWindowViews::NativeWindowViews(const mate::Dictionary& options,
                                     NativeWindow* parent)
    : NativeWindow(options, parent),
      root_view_(new RootView(this)),
      keyboard_event_handler_(new views::UnhandledKeyboardEventHandler) {
  options.Get(options::kTitle, &title_);
  bool menu_bar_autohide;
  if (options.Get(options::kAutoHideMenuBar, &menu_bar_autohide))
    root_view_->SetAutoHideMenuBar(menu_bar_autohide);
#if defined(OS_WIN)
  
  
  options.Get(options::kResizable, &resizable_);
  options.Get(options::kMinimizable, &minimizable_);