* 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 (!thick_frame_)
      frame_style &= ~(WS_THICKFRAME | WS_CAPTION);
    ::SetWindowLong(GetAcceleratedWidget(), GWL_STYLE, frame_style);
  }
  LONG ex_style = ::GetWindowLong(GetAcceleratedWidget(), GWL_EXSTYLE);
  if (window_type == "toolbar")
    ex_style |= WS_EX_TOOLWINDOW;
  ::SetWindowLong(GetAcceleratedWidget(), GWL_EXSTYLE, ex_style);
#endif
  if (has_frame()) {
    
    
    widget()->set_frame_type(views::Widget::FrameType::FRAME_TYPE_FORCE_NATIVE);
    widget()->FrameTypeChanged();
#if defined(OS_WIN)
    
    if (!thick_frame_)
      FlipWindowStyle(GetAcceleratedWidget(), false, WS_THICKFRAME);
#endif
  }
  
  SetContentView(new views::View());
  gfx::Size size = bounds.size();
  if (has_frame() &&
      options.Get(options::kUseContentSize, &use_content_size_) &&
      use_content_size_)
    size = ContentBoundsToWindowBounds(gfx::Rect(size)).size();
  widget()->CenterWindow(size);
#if defined(OS_WIN)