* 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& description) {
  window_->SetOverlayIcon(overlay, description);
}
void TopLevelWindow::SetVisibleOnAllWorkspaces(bool visible,
                                               mate::Arguments* args) {
  mate::Dictionary options;
  bool visibleOnFullScreen = false;
  args->GetNext(&options) &&
      options.Get("visibleOnFullScreen", &visibleOnFullScreen);
  return window_->SetVisibleOnAllWorkspaces(visible, visibleOnFullScreen);
}
bool TopLevelWindow::IsVisibleOnAllWorkspaces() {
  return window_->IsVisibleOnAllWorkspaces();
}
void TopLevelWindow::SetAutoHideCursor(bool auto_hide) {
  window_->SetAutoHideCursor(auto_hide);
}
void TopLevelWindow::SetVibrancy(v8::Isolate* isolate,
                                 v8::Local<v8::Value> value) {
  std::string type = gin::V8ToString(isolate, value);
  window_->SetVibrancy(type);
}
void TopLevelWindow::SetTouchBar(
    const std::vector<mate::PersistentDictionary>& items) {
  window_->SetTouchBar(items);
}
void TopLevelWindow::RefreshTouchBarItem(const std::string& item_id) {
  window_->RefreshTouchBarItem(item_id);
}
void TopLevelWindow::SetEscapeTouchBarItem(