sync: Support non-blocking initial sync in proto

sync/sessions/nudge_tracker.h

Index: sync/sessions/nudge_tracker.h
diff --git a/sync/sessions/nudge_tracker.h b/sync/sessions/nudge_tracker.h
index a602537f1f8c4c3dc03e26689c77bab8efe9754c..0397c5dc3dba42e49f442fd33d766f8d13b75a9b 100644
--- a/sync/sessions/nudge_tracker.h
+++ b/sync/sessions/nudge_tracker.h
@@ -62,6 +62,9 @@ class SYNC_EXPORT_PRIVATE NudgeTracker {
   void RecordRemoteInvalidation(syncer::ModelType type,
                                 scoped_ptr<InvalidationInterface> invalidation);
 
+  // Take note that an initial sync is pending for this type.
+  void RecordInitialSyncRequired(syncer::ModelType type);
+
   // These functions should be called to keep this class informed of the status
   // of the connection to the invalidations server.
   void OnInvalidationsEnabled();