Update ajax_nav.js

I see firefox picks up 
```onpopstate = function (oEvent) {
		bUpdateURL = false;
		oPageInfo.title = oEvent.state.title;
		oPageInfo.url = oEvent.state.url;
		getPage();
	};```

but chrome does not. Adding the 	

```window.onpopstate```

gets it to work in chrome, not sure if this is correct or not.