* 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 (resizable != resizable_) {
    
    
    if (resizable) {
      SetContentSizeConstraints(old_size_constraints_);
      SetMaximizable(maximizable_);
    } else {
      old_size_constraints_ = GetContentSizeConstraints();
      resizable_ = false;
      gfx::Size content_size = GetContentSize();
      SetContentSizeConstraints(
          extensions::SizeConstraints(content_size, content_size));
    }
  }
#if defined(OS_WIN)
  if (has_frame() && thick_frame_)
    FlipWindowStyle(GetAcceleratedWidget(), resizable, WS_THICKFRAME);
#endif
  resizable_ = resizable;
}
#if defined(OS_WIN)
void NativeWindowViews::MoveTop() {
  gfx::Point pos = GetPosition();
  gfx::Size size = GetSize();
  ::SetWindowPos(GetAcceleratedWidget(), HWND_TOP, pos.x(), pos.y(),
                 size.width(), size.height(),
                 SWP_NOACTIVATE | SWP_NOMOVE | SWP_NOSIZE | SWP_SHOWWINDOW);
}
#endif
bool NativeWindowViews::IsResizable() {