* 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_->CloseFilePreview();
}
v8::Local<v8::Value> TopLevelWindow::GetContentView() const {
  if (content_view_.IsEmpty())
    return v8::Null(isolate());
  else
    return v8::Local<v8::Value>::New(isolate(), content_view_);
}
v8::Local<v8::Value> TopLevelWindow::GetParentWindow() const {
  if (parent_window_.IsEmpty())
    return v8::Null(isolate());
  else
    return v8::Local<v8::Value>::New(isolate(), parent_window_);
}
std::vector<v8::Local<v8::Object>> TopLevelWindow::GetChildWindows() const {
  return child_windows_.Values(isolate());
}
v8::Local<v8::Value> TopLevelWindow::GetBrowserView() const {
  if (browser_view_.IsEmpty()) {
    return v8::Null(isolate());
  }
  return v8::Local<v8::Value>::New(isolate(), browser_view_);
}
bool TopLevelWindow::IsModal() const {