* 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 defined(OS_WIN)
  if (has_frame())
    return ::GetWindowLong(GetAcceleratedWidget(), GWL_STYLE) & WS_THICKFRAME;
#endif
  return CanResize();
}
void NativeWindowViews::SetMovable(bool movable) {
  movable_ = movable;
}
bool NativeWindowViews::IsMovable() {
#if defined(OS_WIN)
  return movable_;
#else
  return true;  
#endif
}
void NativeWindowViews::SetMinimizable(bool minimizable) {
#if defined(OS_WIN)
  FlipWindowStyle(GetAcceleratedWidget(), minimizable, WS_MINIMIZEBOX);
#endif
  minimizable_ = minimizable;
}
bool NativeWindowViews::IsMinimizable() {
#if defined(OS_WIN)
  return ::GetWindowLong(GetAcceleratedWidget(), GWL_STYLE) & WS_MINIMIZEBOX;
#else
  return true;  
#endif
}
void NativeWindowViews::SetMaximizable(bool maximizable) {