[traceviewer] Object-Events are not shown if they do not contain a snapshot

[h-mathias]

In process_track_base.html object-events are not drawn (track) if they do not have at least one snapshot.

// Do not create tracks for instances that have no snapshots.
if (instance.snapshots.length === 0) continue;

Not sure if this is really indented because I think it is also useful to track object lifetimes without the need to create snapshots. Removing this code works for me.
Also the Trace Event Format document does not state that a snapshot is required.