* 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.h
bool CanMaximize() const override;
  bool CanMinimize() const override;
  base::string16 GetWindowTitle() const override;
  views::View* GetContentsView() override;
  bool ShouldDescendIntoChildForEventHandling(
      gfx::NativeView child,
      const gfx::Point& location) override;
  views::ClientView* CreateClientView(views::Widget* widget) override;
  views::NonClientFrameView* CreateNonClientFrameView(
      views::Widget* widget) override;
  void OnWidgetMove() override;
#if defined(OS_WIN)
  bool ExecuteWindowsCommand(int command_id) override;
#endif
#if defined(OS_WIN)
  
  bool PreHandleMSG(UINT message,
                    WPARAM w_param,
                    LPARAM l_param,
                    LRESULT* result) override;
  void HandleSizeEvent(WPARAM w_param, LPARAM l_param);
  void SetForwardMouseMessages(bool forward);
  static LRESULT CALLBACK SubclassProc(HWND hwnd,
                                       UINT msg,
                                       WPARAM w_param,
                                       LPARAM l_param,
                                       UINT_PTR subclass_id,
                                       DWORD_PTR ref_data);
  static LRESULT CALLBACK MouseHookProc(int n_code,
                                        WPARAM w_param,