* 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
Emit("maximize");
}
void TopLevelWindow::OnWindowUnmaximize() {
  Emit("unmaximize");
}
void TopLevelWindow::OnWindowMinimize() {
  Emit("minimize");
}
void TopLevelWindow::OnWindowRestore() {
  Emit("restore");
}
void TopLevelWindow::OnWindowWillResize(const gfx::Rect& new_bounds,
                                        bool* prevent_default) {
  if (Emit("will-resize", new_bounds)) {
    *prevent_default = true;
  }
}
void TopLevelWindow::OnWindowResize() {
  Emit("resize");
}
void TopLevelWindow::OnWindowWillMove(const gfx::Rect& new_bounds,
                                      bool* prevent_default) {
  if (Emit("will-move", new_bounds)) {
    *prevent_default = true;
  }
}
void TopLevelWindow::OnWindowMove() {
  Emit("move");
}
void TopLevelWindow::OnWindowMoved() {