* 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 extensions::SizeConstraints GetContentSizeConstraints() const;
  virtual void SetMinimumSize(const gfx::Size& size);
  virtual gfx::Size GetMinimumSize() const;
  virtual void SetMaximumSize(const gfx::Size& size);
  virtual gfx::Size GetMaximumSize() const;
  virtual gfx::Size GetContentMinimumSize() const;
  virtual gfx::Size GetContentMaximumSize() const;
  virtual void SetSheetOffset(const double offsetX, const double offsetY);
  virtual double GetSheetOffsetX();
  virtual double GetSheetOffsetY();
  virtual void SetResizable(bool resizable) = 0;
#if defined(OS_WIN) || defined(OS_MACOSX)
  virtual void MoveTop() = 0;
#endif
  virtual bool IsResizable() = 0;
  virtual void SetMovable(bool movable) = 0;
  virtual bool IsMovable() = 0;
  virtual void SetMinimizable(bool minimizable) = 0;
  virtual bool IsMinimizable() = 0;
  virtual void SetMaximizable(bool maximizable) = 0;
  virtual bool IsMaximizable() = 0;
  virtual void SetFullScreenable(bool fullscreenable) = 0;
  virtual bool IsFullScreenable() = 0;
  virtual void SetClosable(bool closable) = 0;
  virtual bool IsClosable() = 0;
  virtual void SetAlwaysOnTop(bool top,
                              const std::string& level = "floating",
                              int relativeLevel = 0,
                              std::string* error = nullptr) = 0;
  virtual bool IsAlwaysOnTop() = 0;