* 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
if (is_modal() && NativeWindow::parent())
    static_cast<NativeWindowViews*>(parent())->DecrementChildModals();
  widget()->Hide();
  NotifyWindowHide();
#if defined(USE_X11)
  if (global_menu_bar_)
    global_menu_bar_->OnWindowUnmapped();
#endif
}
bool NativeWindowViews::IsVisible() {
  return widget()->IsVisible();
}
bool NativeWindowViews::IsEnabled() {
#if defined(OS_WIN)
  return ::IsWindowEnabled(GetAcceleratedWidget());
#elif defined(USE_X11)
  return !event_disabler_.get();
#endif
}
void NativeWindowViews::IncrementChildModals() {
  num_modal_children_++;
  SetEnabledInternal(ShouldBeEnabled());
}
void NativeWindowViews::DecrementChildModals() {
  if (num_modal_children_ > 0) {
    num_modal_children_--;
  }
  SetEnabledInternal(ShouldBeEnabled());
}
void NativeWindowViews::SetEnabled(bool enable) {