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.
if (aSlice.args !== undefined && aSlice.args.params !== undefined && aSlice.args.params.url !== undefined)
  return aSlice.args.params.url;