* 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) const {
  if (!has_frame())
    return bounds;
  gfx::Rect content_bounds(bounds);
#if defined(OS_WIN)
  HWND hwnd = GetAcceleratedWidget();
  content_bounds.set_size(
      display::win::ScreenWin::DIPToScreenSize(hwnd, content_bounds.size()));
  RECT rect;
  SetRectEmpty(&rect);
  DWORD style = ::GetWindowLong(hwnd, GWL_STYLE);
  DWORD ex_style = ::GetWindowLong(hwnd, GWL_EXSTYLE);
  AdjustWindowRectEx(&rect, style, FALSE, ex_style);
  content_bounds.set_width(content_bounds.width() - (rect.right - rect.left));
  content_bounds.set_height(content_bounds.height() - (rect.bottom - rect.top));
  content_bounds.set_size(
      display::win::ScreenWin::ScreenToDIPSize(hwnd, content_bounds.size()));
#endif
  if (root_view_->HasMenu() && root_view_->IsMenuBarVisible()) {
    int menu_bar_height = root_view_->GetMenuBarHeight();
    content_bounds.set_y(content_bounds.y() + menu_bar_height);
    content_bounds.set_height(content_bounds.height() - menu_bar_height);
  }
  return content_bounds;
}
void NativeWindowViews::UpdateDraggableRegions(