Bug 1033661 - [Collections] Icons overlay "Connect to the Internet" mess...

...age when browsing offline a Smart Collection
I think this is ok, but it seems we're accessing a lot of stuff that wasn't really meant to be exposed. I'm not sure if it's any better, but what if we get the last item from the grid, and add item.x + item.pixelHeight? I think I'm fine with whatever you like better.

```
var items = grid.getItems();
var last = items[items.length - 1];
marginTop = last.y + last.itemHeight;
```