* 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/api/atom_api_top_level_window.cc
const MessageCallback& callback) {
  messages_callback_map_[message] = callback;
  return true;
}
void TopLevelWindow::UnhookWindowMessage(UINT message) {
  if (!ContainsKey(messages_callback_map_, message))
    return;
  messages_callback_map_.erase(message);
}
bool TopLevelWindow::IsWindowMessageHooked(UINT message) {
  return ContainsKey(messages_callback_map_, message);
}
void TopLevelWindow::UnhookAllWindowMessages() {
  messages_callback_map_.clear();
}
bool TopLevelWindow::SetThumbnailClip(const gfx::Rect& region) {
  auto* window = static_cast<NativeWindowViews*>(window_.get());
  return window->taskbar_host().SetThumbnailClip(
      window_->GetAcceleratedWidget(), region);
}
bool TopLevelWindow::SetThumbnailToolTip(const std::string& tooltip) {
  auto* window = static_cast<NativeWindowViews*>(window_.get());
  return window->taskbar_host().SetThumbnailToolTip(
      window_->GetAcceleratedWidget(), tooltip);
}
void TopLevelWindow::SetAppDetails(const mate::Dictionary& options) {