sync: Expose sync functionality as functions

sync/sessions/data_type_tracker.h

Index: sync/sessions/data_type_tracker.h
diff --git a/sync/sessions/data_type_tracker.h b/sync/sessions/data_type_tracker.h
index d6da9462f9f0d499a18fdd414bb68c112f9e520e..9be6d169bbcc6cad61428ffc806f61a5ad5ac1d6 100644
--- a/sync/sessions/data_type_tracker.h
+++ b/sync/sessions/data_type_tracker.h
@@ -59,6 +59,10 @@ class DataTypeTracker {
   // Returns the most recent invalidation payload.
   std::string GetMostRecentInvalidationPayload() const;
 
+  // Fills in the legacy invalidaiton payload information fields.
+  void SetLegacyNotificationHint(
+      sync_pb::DataTypeProgressMarker* progress) const;
+
   // Fills some type-specific contents of a GetUpdates request protobuf.  These
   // messages provide the server with the information it needs to decide how to
   // handle a request.