* 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
constraints.set_minimum_size(min_size);
    }
    if (constraints.HasMaximumSize()) {
      gfx::Size max_size = constraints.GetMaximumSize();
      max_size.set_height(max_size.height() + menu_bar_height);
      constraints.set_maximum_size(max_size);
    }
    SetContentSizeConstraints(constraints);
    
    SetContentSize(content_size);
  }
}
void NativeWindowViews::SetBrowserView(NativeBrowserView* view) {
  if (!content_view())
    return;
  if (browser_view()) {
    content_view()->RemoveChildView(
        browser_view()->GetInspectableWebContentsView()->GetView());
    set_browser_view(nullptr);
  }
  if (!view) {
    return;
  }
  
  
  set_browser_view(view);
  content_view()->AddChildView(
      view->GetInspectableWebContentsView()->GetView());
}
void NativeWindowViews::SetParentWindow(NativeWindow* parent) {