* 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
NativeWindow::SetParentWindow(parent);
#if defined(USE_X11)
  XDisplay* xdisplay = gfx::GetXDisplay();
  XSetTransientForHint(
      xdisplay, GetAcceleratedWidget(),
      parent ? parent->GetAcceleratedWidget() : DefaultRootWindow(xdisplay));
#elif defined(OS_WIN) && defined(DEBUG)
  
  
  if (parent) {
    ::SetParent(GetAcceleratedWidget(), parent->GetAcceleratedWidget());
    views::Widget::ReparentNativeView(GetNativeWindow(),
                                      parent->GetNativeWindow());
    wm::AddTransientChild(parent->GetNativeWindow(), GetNativeWindow());
  } else {
    if (!GetNativeWindow()->parent())
      return;
    ::SetParent(GetAcceleratedWidget(), NULL);
    views::Widget::ReparentNativeView(GetNativeWindow(), nullptr);
    wm::RemoveTransientChild(GetNativeWindow()->parent(), GetNativeWindow());
  }
#endif
}
gfx::NativeView NativeWindowViews::GetNativeView() const {