* 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
return window_->is_modal();
}
bool TopLevelWindow::SetThumbarButtons(mate::Arguments* args) {
#if defined(OS_WIN)
  std::vector<TaskbarHost::ThumbarButton> buttons;
  if (!args->GetNext(&buttons)) {
    args->ThrowError();
    return false;
  }
  auto* window = static_cast<NativeWindowViews*>(window_.get());
  return window->taskbar_host().SetThumbarButtons(
      window_->GetAcceleratedWidget(), buttons);
#else
  return false;
#endif
}
#if defined(TOOLKIT_VIEWS)
void TopLevelWindow::SetIcon(mate::Handle<NativeImage> icon) {
#if defined(OS_WIN)
  static_cast<NativeWindowViews*>(window_.get())
      ->SetIcon(icon->GetHICON(GetSystemMetrics(SM_CXSMICON)),
                icon->GetHICON(GetSystemMetrics(SM_CXICON)));
#elif defined(USE_X11)
  static_cast<NativeWindowViews*>(window_.get())
      ->SetIcon(icon->image().AsImageSkia());
#endif
}
#endif
#if defined(OS_WIN)
bool TopLevelWindow::HookWindowMessage(UINT message,