Add an action for server to send down rollback command.

sync/engine/syncer_proto_util.cc

Index: sync/engine/syncer_proto_util.cc
diff --git a/sync/engine/syncer_proto_util.cc b/sync/engine/syncer_proto_util.cc
index dab228f3ae303df826bfa9ca73458dd5b2b046c6..d717948de9a947d33d489ea34512acd7c7bd2cbc 100644
--- a/sync/engine/syncer_proto_util.cc
+++ b/sync/engine/syncer_proto_util.cc
@@ -122,6 +122,8 @@ SyncProtocolErrorType ConvertSyncProtocolErrorTypePBToLocalType(
       return MIGRATION_DONE;
     case sync_pb::SyncEnums::DISABLED_BY_ADMIN:
       return DISABLED_BY_ADMIN;
+    case sync_pb::SyncEnums::USER_ROLLBACK:
+      return USER_ROLLBACK;
     case sync_pb::SyncEnums::UNKNOWN:
       return UNKNOWN_ERROR;
     case sync_pb::SyncEnums::USER_NOT_ACTIVATED:
@@ -331,7 +333,9 @@ SyncProtocolError ConvertLegacyErrorCodeToNewError(
     error.action = DISABLE_SYNC_ON_CLIENT;
   } else if (error_type == sync_pb::SyncEnums::DISABLED_BY_ADMIN) {
     error.action = STOP_SYNC_FOR_DISABLED_ACCOUNT;
-  }  // There is no other action we can compute for legacy server.
+  } else if (error_type == sync_pb::SyncEnums::USER_ROLLBACK) {
+    error.action = DISABLE_SYNC_AND_ROLLBACK;
+  } // There is no other action we can compute for legacy server.
   return error;
 }