* 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
window_->SetAutoHideMenuBar(auto_hide);
}
bool TopLevelWindow::IsMenuBarAutoHide() {
  return window_->IsMenuBarAutoHide();
}
void TopLevelWindow::SetMenuBarVisibility(bool visible) {
  window_->SetMenuBarVisibility(visible);
}
bool TopLevelWindow::IsMenuBarVisible() {
  return window_->IsMenuBarVisible();
}
void TopLevelWindow::SetAspectRatio(double aspect_ratio,
                                    mate::Arguments* args) {
  gfx::Size extra_size;
  args->GetNext(&extra_size);
  window_->SetAspectRatio(aspect_ratio, extra_size);
}
void TopLevelWindow::PreviewFile(const std::string& path,
                                 mate::Arguments* args) {
  std::string display_name;
  if (!args->GetNext(&display_name))
    display_name = path;
  window_->PreviewFile(path, display_name);
}
void TopLevelWindow::CloseFilePreview() {