* 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/native_window_views.cc
#if defined(USE_X11)
  if (menu_model == nullptr)
    global_menu_bar_.reset();
  if (!global_menu_bar_ && ShouldUseGlobalMenuBar())
    global_menu_bar_.reset(new GlobalMenuBarX11(this));
  
  if (global_menu_bar_ && global_menu_bar_->IsServerStarted()) {
    global_menu_bar_->SetMenu(menu_model);
    return;
  }
#endif
  
  gfx::Size content_size = GetContentSize();
  bool should_reset_size = use_content_size_ && has_frame() &&
                           !IsMenuBarAutoHide() &&
                           ((!!menu_model) != root_view_->HasMenu());
  root_view_->SetMenu(menu_model);
  if (should_reset_size) {
    
    int menu_bar_height = root_view_->GetMenuBarHeight();
    extensions::SizeConstraints constraints = GetContentSizeConstraints();
    if (constraints.HasMinimumSize()) {
      gfx::Size min_size = constraints.GetMinimumSize();
      min_size.set_height(min_size.height() + menu_bar_height);