* 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/native_window_mac.h
bool is_kiosk_ = false;
  bool was_fullscreen_ = false;
  bool zoom_to_page_width_ = false;
  bool fullscreen_window_title_ = false;
  bool resizable_ = true;
  NSInteger attention_request_id_ = 0;  
  
  NSApplicationPresentationOptions kiosk_options_;
  
  TitleBarStyle title_bar_style_ = NORMAL;
  
  
  base::Optional<bool> window_button_visibility_;
  
  bool always_simple_fullscreen_ = false;
  bool is_simple_fullscreen_ = false;
  bool was_maximizable_ = false;
  bool was_movable_ = false;
  NSRect original_frame_;
  NSInteger original_level_;
  NSUInteger simple_fullscreen_mask_;
  base::scoped_nsobject<NSColor> background_color_before_vibrancy_;
  bool transparency_before_vibrancy_ = false;
  
  NSApplicationPresentationOptions simple_fullscreen_options_;
  DISALLOW_COPY_AND_ASSIGN(NativeWindowMac);
};
}  
#endif