* 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
const std::string& item_id,
    const base::DictionaryValue& details) {
  Emit("-touch-bar-interaction", item_id, details);
}
void TopLevelWindow::OnNewWindowForTab() {
  Emit("new-window-for-tab");
}
#if defined(OS_WIN)
void TopLevelWindow::OnWindowMessage(UINT message,
                                     WPARAM w_param,
                                     LPARAM l_param) {
  if (IsWindowMessageHooked(message)) {
    messages_callback_map_[message].Run(
        ToBuffer(isolate(), static_cast<void*>(&w_param), sizeof(WPARAM)),
        ToBuffer(isolate(), static_cast<void*>(&l_param), sizeof(LPARAM)));
  }
}
#endif
void TopLevelWindow::SetContentView(mate::Handle<View> view) {
  content_view_.Reset(isolate(), view.ToV8());
  window_->SetContentView(view->view());
}
void TopLevelWindow::Close() {
  window_->Close();
}
void TopLevelWindow::Focus() {
  window_->Focus(true);
}
void TopLevelWindow::Blur() {