* 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
return GetNativeWindow()->GetHost()->GetAcceleratedWidget();
}
gfx::Rect NativeWindowViews::ContentBoundsToWindowBounds(
    const gfx::Rect& bounds) const {
  if (!has_frame())
    return bounds;
  gfx::Rect window_bounds(bounds);
#if defined(OS_WIN)
  HWND hwnd = GetAcceleratedWidget();
  gfx::Rect dpi_bounds = display::win::ScreenWin::DIPToScreenRect(hwnd, bounds);
  window_bounds = display::win::ScreenWin::ScreenToDIPRect(
      hwnd,
      widget()->non_client_view()->GetWindowBoundsForClientBounds(dpi_bounds));
#endif
  if (root_view_->HasMenu() && root_view_->IsMenuBarVisible()) {
    int menu_bar_height = root_view_->GetMenuBarHeight();
    window_bounds.set_y(window_bounds.y() - menu_bar_height);
    window_bounds.set_height(window_bounds.height() + menu_bar_height);
  }
  return window_bounds;
}
gfx::Rect NativeWindowViews::WindowBoundsToContentBounds(