* 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
ResetBrowserView();
  mate::Handle<BrowserView> browser_view;
  if (value->IsNull() || value->IsUndefined()) {
    window_->SetBrowserView(nullptr);
  } else if (mate::ConvertFromV8(isolate(), value, &browser_view)) {
    window_->SetBrowserView(browser_view->view());
    browser_view->web_contents()->SetOwnerWindow(window_.get());
    browser_view_.Reset(isolate(), value);
  }
}
v8::Local<v8::Value> TopLevelWindow::GetNativeWindowHandle() {
  gfx::AcceleratedWidget handle = window_->GetAcceleratedWidget();
  return ToBuffer(isolate(), static_cast<void*>(&handle), sizeof(handle));
}
void TopLevelWindow::SetProgressBar(double progress, mate::Arguments* args) {
  mate::Dictionary options;
  std::string mode;
  args->GetNext(&options) && options.Get("mode", &mode);
  NativeWindow::ProgressState state = NativeWindow::PROGRESS_NORMAL;
  if (mode == "error")
    state = NativeWindow::PROGRESS_ERROR;
  else if (mode == "paused")
    state = NativeWindow::PROGRESS_PAUSED;
  else if (mode == "indeterminate")
    state = NativeWindow::PROGRESS_INDETERMINATE;
  else if (mode == "none")
    state = NativeWindow::PROGRESS_NONE;
  window_->SetProgressBar(progress, state);
}
void TopLevelWindow::SetOverlayIcon(const gfx::Image& overlay,