* Output of `node_modules/.bin/electron --version`:
v3.0.6
v4.0.0-beta.5

* Operating System (Platform and Version):
macOS High Siearra 10.13.6

* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable):
v2.0.11

**Expected Behavior**
BrowserWindow.getNativeWindowHandle() must return non-nullable result.

**Actual behavior**
BrowserWindow.getNativeWindowHandle() returns null.

**To Reproduce**

***Sample code***
```
const { app, BrowserWindow } = require('electron');

let win

app
	.on('ready', () => {
		win = new BrowserWindow({width: 800, height: 600})
		win.loadURL('https://www.google.com')
		
		win.on('closed', () => win = null);

		const handle = win.getNativeWindowHandle();
		console.log('Current window: ' + handle.toString('hex'));
	})
	.on('window-all-closed', () => app.quit());
```

***Expected output***
Something different than `Current window: 0000000000000000`

***Actual output***
`Current window: 0000000000000000`
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) {