sync: Introduce control data types

sync/syncable/model_type.cc

Index: sync/syncable/model_type.cc
diff --git a/sync/syncable/model_type.cc b/sync/syncable/model_type.cc
index 83e803cdd58c451f6b8eae1efa8e35e56e156203..51647958763d3932593aae2441324aa86fd5a8c0 100644
--- a/sync/syncable/model_type.cc
+++ b/sync/syncable/model_type.cc
@@ -226,6 +226,22 @@ bool ShouldMaintainPosition(ModelType model_type) {
   return model_type == BOOKMARKS;
 }
 
+bool IsControlType(ModelType model_type) {
+  return ControlTypes().Has(model_type);
+}
+
+ModelTypeSet ControlTypes() {
+  ModelTypeSet set;
+  set.Put(NIGORI);
+  return set;
+}
+
+ModelTypeSet EarlyDownloadTypes() {
+  ModelTypeSet set;
+  set.Put(NIGORI);
+  return set;
+}
+
 const char* ModelTypeToString(ModelType model_type) {
   // This is used in serialization routines as well as for displaying debug
   // information.  Do not attempt to change these string values unless you know