* 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.h
virtual void ShowInactive() = 0;
  virtual void Hide() = 0;
  virtual bool IsVisible() = 0;
  virtual bool IsEnabled() = 0;
  virtual void SetEnabled(bool enable) = 0;
  virtual void Maximize() = 0;
  virtual void Unmaximize() = 0;
  virtual bool IsMaximized() = 0;
  virtual void Minimize() = 0;
  virtual void Restore() = 0;
  virtual bool IsMinimized() = 0;
  virtual void SetFullScreen(bool fullscreen) = 0;
  virtual bool IsFullscreen() const = 0;
  virtual void SetBounds(const gfx::Rect& bounds, bool animate = false) = 0;
  virtual gfx::Rect GetBounds() = 0;
  virtual void SetSize(const gfx::Size& size, bool animate = false);
  virtual gfx::Size GetSize();
  virtual void SetPosition(const gfx::Point& position, bool animate = false);
  virtual gfx::Point GetPosition();
  virtual void SetContentSize(const gfx::Size& size, bool animate = false);
  virtual gfx::Size GetContentSize();
  virtual void SetContentBounds(const gfx::Rect& bounds, bool animate = false);
  virtual gfx::Rect GetContentBounds();
  virtual bool IsNormal();
  virtual gfx::Rect GetNormalBounds() = 0;
  virtual void SetSizeConstraints(
      const extensions::SizeConstraints& size_constraints);
  virtual extensions::SizeConstraints GetSizeConstraints() const;
  virtual void SetContentSizeConstraints(
      const extensions::SizeConstraints& size_constraints);