* 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
v8::Local<v8::Object> wrapper,
                               const mate::Dictionary& options)
    : TopLevelWindow(isolate, options) {
  InitWith(isolate, wrapper);
  
  window()->InitFromOptions(options);
}
TopLevelWindow::~TopLevelWindow() {
  if (!window_->IsClosed())
    window_->CloseImmediately();
  
  
  base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, window_.release());
}
void TopLevelWindow::InitWith(v8::Isolate* isolate,
                              v8::Local<v8::Object> wrapper) {
  AttachAsUserData(window_.get());
  mate::TrackableObject<TopLevelWindow>::InitWith(isolate, wrapper);
  
  
  if (!parent_window_.IsEmpty()) {
    mate::Handle<TopLevelWindow> parent;
    mate::ConvertFromV8(isolate, GetParentWindow(), &parent);
    DCHECK(!parent.IsEmpty());
    parent->child_windows_.Set(isolate, weak_map_id(), wrapper);
  }
}
void TopLevelWindow::WillCloseWindow(bool* prevent_default) {