sync: Introduce control data types

sync/sessions/sync_session.cc

Index: sync/sessions/sync_session.cc
diff --git a/sync/sessions/sync_session.cc b/sync/sessions/sync_session.cc
index ee296682111b4449f3c6f9e2b59b20ea579f5354..e65466bece1879858a066a69b19f2f4ac49e73a4 100644
--- a/sync/sessions/sync_session.cc
+++ b/sync/sessions/sync_session.cc
@@ -27,9 +27,11 @@ std::set<ModelSafeGroup> ComputeEnabledGroups(
        it != routing_info.end(); ++it) {
     enabled_groups.insert(it->second);
   }
-  // GROUP_PASSIVE is always enabled, since that's the group that
-  // top-level folders map to.
+  // GROUP_PASSIVE is always enabled, since that's the group that top-level
+  // folders map to.
+  // GROUP_CONTROL manages some meta-data types that are always enabled.
   enabled_groups.insert(GROUP_PASSIVE);
+  enabled_groups.insert(GROUP_CONTROL);
   if (DCHECK_IS_ON()) {
     // We sometimes create dummy SyncSession objects (see
     // SyncScheduler::InitialSnapshot) so don't check in that case.