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.
Can you inherit from tracing.tracks.SliceTrack instead of RectTrack? Then you only need to override set slices() and the rest can be left for the parent class.