* 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 (enable != is_enabled_) {
    is_enabled_ = enable;
    SetEnabledInternal(ShouldBeEnabled());
  }
}
bool NativeWindowViews::ShouldBeEnabled() {
  return is_enabled_ && (num_modal_children_ == 0);
}
void NativeWindowViews::SetEnabledInternal(bool enable) {
  if (enable && IsEnabled()) {
    return;
  } else if (!enable && !IsEnabled()) {
    return;
  }
#if defined(OS_WIN)
  ::EnableWindow(GetAcceleratedWidget(), enable);
#elif defined(USE_X11)
  views::DesktopWindowTreeHostX11* tree_host =
      views::DesktopWindowTreeHostX11::GetHostForXID(GetAcceleratedWidget());
  if (enable) {
    tree_host->RemoveEventRewriter(event_disabler_.get());
    event_disabler_.reset();
  } else {
    event_disabler_.reset(new EventDisabler);
    tree_host->AddEventRewriter(event_disabler_.get());
  }
#endif
}
void NativeWindowViews::Maximize() {