* 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 mate::PersistentDictionary& item) {
  window_->SetEscapeTouchBarItem(item);
}
void TopLevelWindow::SelectPreviousTab() {
  window_->SelectPreviousTab();
}
void TopLevelWindow::SelectNextTab() {
  window_->SelectNextTab();
}
void TopLevelWindow::MergeAllWindows() {
  window_->MergeAllWindows();
}
void TopLevelWindow::MoveTabToNewWindow() {
  window_->MoveTabToNewWindow();
}
void TopLevelWindow::ToggleTabBar() {
  window_->ToggleTabBar();
}
void TopLevelWindow::AddTabbedWindow(NativeWindow* window,
                                     mate::Arguments* args) {
  if (!window_->AddTabbedWindow(window))
    args->ThrowError("AddTabbedWindow cannot be called by a window on itself.");
}
void TopLevelWindow::SetWindowButtonVisibility(bool visible,
                                               mate::Arguments* args) {
  if (!window_->SetWindowButtonVisibility(visible)) {
    args->ThrowError("Not supported for this window");
  }
}
void TopLevelWindow::SetAutoHideMenuBar(bool auto_hide) {