* 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_->IsAlwaysOnTop();
}
void TopLevelWindow::Center() {
  window_->Center();
}
void TopLevelWindow::SetPosition(int x, int y, mate::Arguments* args) {
  bool animate = false;
  args->GetNext(&animate);
  window_->SetPosition(gfx::Point(x, y), animate);
}
std::vector<int> TopLevelWindow::GetPosition() {
  std::vector<int> result(2);
  gfx::Point pos = window_->GetPosition();
  result[0] = pos.x();
  result[1] = pos.y();
  return result;
}
#if defined(OS_WIN) || defined(OS_MACOSX)
void TopLevelWindow::MoveTop() {
  window_->MoveTop();
}
#endif
void TopLevelWindow::SetTitle(const std::string& title) {