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 65d5926bd7d93c62c592fdcd2058c700c90b1b0f..6488f8e105de4b8e21fd289d849378c152da39b7 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.