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.
@natduca  says we should try not to modify the model though..
but trying to set/modify the title of slice does sound easier than this.