* 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/api/atom_api_top_level_window.cc
if (parent_window_.IsEmpty())
    return;
  mate::Handle<TopLevelWindow> parent;
  if (!mate::ConvertFromV8(isolate(), GetParentWindow(), &parent) ||
      parent.IsEmpty()) {
    return;
  }
  parent->child_windows_.Remove(weak_map_id());
}
mate::WrappableBase* TopLevelWindow::New(mate::Arguments* args) {
  mate::Dictionary options;
  if (!(args->Length() == 1 && args->GetNext(&options)))
    options = mate::Dictionary::CreateEmpty(args->isolate());
  return new TopLevelWindow(args->isolate(), args->GetThis(), options);
}
void TopLevelWindow::BuildPrototype(v8::Isolate* isolate,
                                    v8::Local<v8::FunctionTemplate> prototype) {
  prototype->SetClassName(mate::StringToV8(isolate, "TopLevelWindow"));
  mate::ObjectTemplateBuilder(isolate, prototype->PrototypeTemplate())
      .MakeDestroyable()
      .SetMethod("setContentView", &TopLevelWindow::SetContentView)
      .SetMethod("close", &TopLevelWindow::Close)
      .SetMethod("focus", &TopLevelWindow::Focus)
      .SetMethod("blur", &TopLevelWindow::Blur)
      .SetMethod("isFocused", &TopLevelWindow::IsFocused)
      .SetMethod("show", &TopLevelWindow::Show)
      .SetMethod("showInactive", &TopLevelWindow::ShowInactive)
      .SetMethod("hide", &TopLevelWindow::Hide)
      .SetMethod("isVisible", &TopLevelWindow::IsVisible)