* 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
.SetMethod("selectPreviousTab", &TopLevelWindow::SelectPreviousTab)
      .SetMethod("selectNextTab", &TopLevelWindow::SelectNextTab)
      .SetMethod("mergeAllWindows", &TopLevelWindow::MergeAllWindows)
      .SetMethod("moveTabToNewWindow", &TopLevelWindow::MoveTabToNewWindow)
      .SetMethod("toggleTabBar", &TopLevelWindow::ToggleTabBar)
      .SetMethod("addTabbedWindow", &TopLevelWindow::AddTabbedWindow)
      .SetMethod("setWindowButtonVisibility",
                 &TopLevelWindow::SetWindowButtonVisibility)
#endif
      .SetMethod("setAutoHideMenuBar", &TopLevelWindow::SetAutoHideMenuBar)
      .SetMethod("isMenuBarAutoHide", &TopLevelWindow::IsMenuBarAutoHide)
      .SetMethod("setMenuBarVisibility", &TopLevelWindow::SetMenuBarVisibility)
      .SetMethod("isMenuBarVisible", &TopLevelWindow::IsMenuBarVisible)
      .SetMethod("setAspectRatio", &TopLevelWindow::SetAspectRatio)
      .SetMethod("previewFile", &TopLevelWindow::PreviewFile)
      .SetMethod("closeFilePreview", &TopLevelWindow::CloseFilePreview)
      .SetMethod("getContentView", &TopLevelWindow::GetContentView)
      .SetMethod("getParentWindow", &TopLevelWindow::GetParentWindow)
      .SetMethod("getChildWindows", &TopLevelWindow::GetChildWindows)
      .SetMethod("getBrowserView", &TopLevelWindow::GetBrowserView)
      .SetMethod("isModal", &TopLevelWindow::IsModal)
      .SetMethod("setThumbarButtons", &TopLevelWindow::SetThumbarButtons)
#if defined(TOOLKIT_VIEWS)
      .SetMethod("setIcon", &TopLevelWindow::SetIcon)
#endif
#if defined(OS_WIN)
      .SetMethod("hookWindowMessage", &TopLevelWindow::HookWindowMessage)
      .SetMethod("isWindowMessageHooked",
                 &TopLevelWindow::IsWindowMessageHooked)
      .SetMethod("unhookWindowMessage", &TopLevelWindow::UnhookWindowMessage)