* 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
widget()->RemoveObserver(this);
#if defined(OS_WIN)
  
  SetForwardMouseMessages(false);
#endif
#if defined(OS_LINUX)
  aura::Window* window = GetNativeWindow();
  if (window)
    window->RemovePreTargetHandler(this);
#endif
}
void NativeWindowViews::SetContentView(views::View* view) {
  if (content_view()) {
    root_view_->RemoveChildView(content_view());
    if (browser_view()) {
      content_view()->RemoveChildView(
          browser_view()->GetInspectableWebContentsView()->GetView());
      set_browser_view(nullptr);
    }
  }
  set_content_view(view);
  focused_view_ = view;
  root_view_->AddChildView(content_view());
  root_view_->Layout();
}
void NativeWindowViews::Close() {