* 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
const gfx::Rect& bounds) {
  if (changed_widget != widget())
    return;
  
  
  const auto new_bounds = GetBounds();
  if (widget_size_ != new_bounds.size()) {
    if (browser_view()) {
      const auto flags = static_cast<NativeBrowserViewViews*>(browser_view())
                             ->GetAutoResizeFlags();
      int width_delta = 0;
      int height_delta = 0;
      if (flags & kAutoResizeWidth) {
        width_delta = new_bounds.width() - widget_size_.width();
      }
      if (flags & kAutoResizeHeight) {
        height_delta = new_bounds.height() - widget_size_.height();
      }
      auto* view = browser_view()->GetInspectableWebContentsView()->GetView();
      auto new_view_size = view->size();
      new_view_size.set_width(new_view_size.width() + width_delta);
      new_view_size.set_height(new_view_size.height() + height_delta);
      view->SetSize(new_view_size);
    }
    NotifyWindowResize();
    widget_size_ = new_bounds.size();
  }
}
void NativeWindowViews::DeleteDelegate() {