* 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
NativeWindow* parent() const { return parent_; }
  bool is_modal() const { return is_modal_; }
 protected:
  NativeWindow(const mate::Dictionary& options, NativeWindow* parent);
  
  views::Widget* GetWidget() override;
  const views::Widget* GetWidget() const override;
  void set_content_view(views::View* view) { content_view_ = view; }
  void set_browser_view(NativeBrowserView* browser_view) {
    browser_view_ = browser_view;
  }
 private:
  std::unique_ptr<views::Widget> widget_;
  
  views::View* content_view_ = nullptr;
  
  bool has_frame_ = true;
  
  bool transparent_ = false;
  
  extensions::SizeConstraints size_constraints_;
  
  bool enable_larger_than_screen_ = false;
  
  bool is_closed_ = false;
  
  
  double sheet_offset_x_ = 0.0;
  double sheet_offset_y_ = 0.0;