* 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
#endif
      .SetMethod("isMovable", &TopLevelWindow::IsMovable)
      .SetMethod("setMinimizable", &TopLevelWindow::SetMinimizable)
      .SetMethod("isMinimizable", &TopLevelWindow::IsMinimizable)
      .SetMethod("setMaximizable", &TopLevelWindow::SetMaximizable)
      .SetMethod("isMaximizable", &TopLevelWindow::IsMaximizable)
      .SetMethod("setFullScreenable", &TopLevelWindow::SetFullScreenable)
      .SetMethod("isFullScreenable", &TopLevelWindow::IsFullScreenable)
      .SetMethod("setClosable", &TopLevelWindow::SetClosable)
      .SetMethod("isClosable", &TopLevelWindow::IsClosable)
      .SetMethod("setAlwaysOnTop", &TopLevelWindow::SetAlwaysOnTop)
      .SetMethod("isAlwaysOnTop", &TopLevelWindow::IsAlwaysOnTop)
      .SetMethod("center", &TopLevelWindow::Center)
      .SetMethod("setPosition", &TopLevelWindow::SetPosition)
      .SetMethod("getPosition", &TopLevelWindow::GetPosition)
      .SetMethod("setTitle", &TopLevelWindow::SetTitle)
      .SetMethod("getTitle", &TopLevelWindow::GetTitle)
      .SetMethod("flashFrame", &TopLevelWindow::FlashFrame)
      .SetMethod("setSkipTaskbar", &TopLevelWindow::SetSkipTaskbar)
      .SetMethod("setSimpleFullScreen", &TopLevelWindow::SetSimpleFullScreen)
      .SetMethod("isSimpleFullScreen", &TopLevelWindow::IsSimpleFullScreen)
      .SetMethod("setKiosk", &TopLevelWindow::SetKiosk)
      .SetMethod("isKiosk", &TopLevelWindow::IsKiosk)
      .SetMethod("setBackgroundColor", &TopLevelWindow::SetBackgroundColor)
      .SetMethod("setHasShadow", &TopLevelWindow::SetHasShadow)
      .SetMethod("hasShadow", &TopLevelWindow::HasShadow)
      .SetMethod("setOpacity", &TopLevelWindow::SetOpacity)
      .SetMethod("getOpacity", &TopLevelWindow::GetOpacity)
      .SetMethod("setShape", &TopLevelWindow::SetShape)
      .SetMethod("setRepresentedFilename",