* 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_views.h
bool IsEnabled() override;
  void SetEnabled(bool enable) override;
  void Maximize() override;
  void Unmaximize() override;
  bool IsMaximized() override;
  void Minimize() override;
  void Restore() override;
  bool IsMinimized() override;
  void SetFullScreen(bool fullscreen) override;
  bool IsFullscreen() const override;
  void SetBounds(const gfx::Rect& bounds, bool animate) override;
  gfx::Rect GetBounds() override;
  gfx::Rect GetContentBounds() override;
  gfx::Size GetContentSize() override;
  gfx::Rect GetNormalBounds() override;
  void SetContentSizeConstraints(
      const extensions::SizeConstraints& size_constraints) override;
  void SetResizable(bool resizable) override;
#if defined(OS_WIN)
  void MoveTop() override;
#endif
  bool IsResizable() override;
  void SetMovable(bool movable) override;
  bool IsMovable() override;
  void SetMinimizable(bool minimizable) override;
  bool IsMinimizable() override;
  void SetMaximizable(bool maximizable) override;
  bool IsMaximizable() override;
  void SetFullScreenable(bool fullscreenable) override;
  bool IsFullScreenable() override;