* 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
&TopLevelWindow::SetRepresentedFilename)
      .SetMethod("getRepresentedFilename",
                 &TopLevelWindow::GetRepresentedFilename)
      .SetMethod("setDocumentEdited", &TopLevelWindow::SetDocumentEdited)
      .SetMethod("isDocumentEdited", &TopLevelWindow::IsDocumentEdited)
      .SetMethod("setIgnoreMouseEvents", &TopLevelWindow::SetIgnoreMouseEvents)
      .SetMethod("setContentProtection", &TopLevelWindow::SetContentProtection)
      .SetMethod("setFocusable", &TopLevelWindow::SetFocusable)
      .SetMethod("setMenu", &TopLevelWindow::SetMenu)
#if !defined(OS_WIN)
      .SetMethod("setParentWindow", &TopLevelWindow::SetParentWindow)
#endif
      .SetMethod("setBrowserView", &TopLevelWindow::SetBrowserView)
      .SetMethod("getNativeWindowHandle",
                 &TopLevelWindow::GetNativeWindowHandle)
      .SetMethod("setProgressBar", &TopLevelWindow::SetProgressBar)
      .SetMethod("setOverlayIcon", &TopLevelWindow::SetOverlayIcon)
      .SetMethod("setVisibleOnAllWorkspaces",
                 &TopLevelWindow::SetVisibleOnAllWorkspaces)
      .SetMethod("isVisibleOnAllWorkspaces",
                 &TopLevelWindow::IsVisibleOnAllWorkspaces)
#if defined(OS_MACOSX)
      .SetMethod("setAutoHideCursor", &TopLevelWindow::SetAutoHideCursor)
#endif
      .SetMethod("setVibrancy", &TopLevelWindow::SetVibrancy)
      .SetMethod("_setTouchBarItems", &TopLevelWindow::SetTouchBar)
      .SetMethod("_refreshTouchBarItem", &TopLevelWindow::RefreshTouchBarItem)
      .SetMethod("_setEscapeTouchBarItem",
                 &TopLevelWindow::SetEscapeTouchBarItem)
#if defined(OS_MACOSX)