There are three kinds of tab restores:
1. Those from a previous session of this device (session restore)
2. Those coming from another device (open a foreign tab)
3. Those previously closed on this device (opening a recently closed tab)

For task id tracking, #1 is probably the most common, and hence the most important. It requires doing a match between the currently open tabs and the previously synced open tabs for this device. Now that  bug 639009  is fixed (restoring previously synced open tabs into the SyncedSessionTracker on startup), this should be feasible, but might require that on Desktop we start persisting the tab sync id (see TabContentsSyncedTabDelegate::[Set/Get]SyncId).

#2 should be relatively straightforward, as Sync is the source of the data to be restored.

#3 might involve some complexity, unless, we start storing the task id itself in the SessionTab object that is used for restoring?