Expose URL as the title of toplevel async slices

Exposing the url param as the title of the toplevel slice improves readability of netlog events. This change adds an async_slice_track that processes each toplevel async slice to find the url param in itself or its descendants if there is a url param, and replaces the title with this url param.
I don't think you want slice here do you? We are the slice, so this?