* 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
#if defined(OS_WIN)
namespace mate {
template <>
struct Converter<atom::TaskbarHost::ThumbarButton> {
  static bool FromV8(v8::Isolate* isolate,
                     v8::Handle<v8::Value> val,
                     atom::TaskbarHost::ThumbarButton* out) {
    mate::Dictionary dict;
    if (!ConvertFromV8(isolate, val, &dict))
      return false;
    dict.Get("click", &(out->clicked_callback));
    dict.Get("tooltip", &(out->tooltip));
    dict.Get("flags", &out->flags);
    return dict.Get("icon", &(out->icon));
  }
};
}  
#endif
namespace atom {
namespace api {
namespace {
v8::Local<v8::Value> ToBuffer(v8::Isolate* isolate, void* val, int size) {
  auto buffer = node::Buffer::Copy(isolate, static_cast<char*>(val), size);
  if (buffer.IsEmpty())
    return v8::Null(isolate);
  else
    return buffer.ToLocalChecked();
}
}  
TopLevelWindow::TopLevelWindow(v8::Isolate* isolate,