* 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_->SetMaximizable(maximizable);
}
bool TopLevelWindow::IsMaximizable() {
  return window_->IsMaximizable();
}
void TopLevelWindow::SetFullScreenable(bool fullscreenable) {
  window_->SetFullScreenable(fullscreenable);
}
bool TopLevelWindow::IsFullScreenable() {
  return window_->IsFullScreenable();
}
void TopLevelWindow::SetClosable(bool closable) {
  window_->SetClosable(closable);
}
bool TopLevelWindow::IsClosable() {
  return window_->IsClosable();
}
void TopLevelWindow::SetAlwaysOnTop(bool top, mate::Arguments* args) {
  std::string level = "floating";
  int relative_level = 0;
  args->GetNext(&level);
  args->GetNext(&relative_level);
  std::string error;
  window_->SetAlwaysOnTop(top, level, relative_level, &error);
  if (!error.empty())
    args->ThrowError(error);
}
bool TopLevelWindow::IsAlwaysOnTop() {