* 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
Emit("always-on-top-changed", IsAlwaysOnTop());
}
void TopLevelWindow::OnExecuteWindowsCommand(const std::string& command_name) {
  Emit("app-command", command_name);
}
void TopLevelWindow::OnTouchBarItemResult(
    const std::string& item_id,
    const base::DictionaryValue& details) {
  Emit("-touch-bar-interaction", item_id, details);
}
void TopLevelWindow::OnNewWindowForTab() {
  Emit("new-window-for-tab");
}
#if defined(OS_WIN)
void TopLevelWindow::OnWindowMessage(UINT message,
                                     WPARAM w_param,
                                     LPARAM l_param) {
  if (IsWindowMessageHooked(message)) {
    messages_callback_map_[message].Run(
        ToBuffer(isolate(), static_cast<void*>(&w_param), sizeof(WPARAM)),
        ToBuffer(isolate(), static_cast<void*>(&l_param), sizeof(LPARAM)));
  }
}
#endif
void TopLevelWindow::SetContentView(mate::Handle<View> view) {
  content_view_.Reset(isolate(), view.ToV8());
  window_->SetContentView(view->view());
}
void TopLevelWindow::Close() {