Add an action for server to send down rollback command.

sync/protocol/sync_enums.proto

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Sync protocol for communication between sync client and server.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 80 matching lines @@
     CLEAR_PENDING      = 7;   // A clear of the user data is pending (e.g.
                               // initiated by privacy request).  Client should
                               // come back later.
     TRANSIENT_ERROR    = 8;   // A transient error occured (eg. backend
                               // timeout). Client should try again later.
     MIGRATION_DONE     = 9;   // Migration has finished for one or more data
                               // types.  Client should clear the cache for
                               // these data types only and then re-sync with
                               // a server.
     DISABLED_BY_ADMIN = 10;   // An administrator disabled sync for this domain.
+    USER_ROLLBACK     = 11;   // Client told to stop syncing and roll back.
     UNKNOWN            = 100; // Unknown value. This should never be explicitly
                               // used; it is the default value when an
                               // out-of-date client parses a value it doesn't
                               // recognize.
   }
 
   enum Action {
     UPGRADE_CLIENT              = 0; // Upgrade the client to latest version.
     CLEAR_USER_DATA_AND_RESYNC  = 1; // Clear user data from dashboard and
                                      // setup sync again.
@@ skipping 40 matching lines @@
                            // confused with FIRST_UPDATE.
     RECONFIGURATION = 10;  // The client is in configuration mode because the
                            // user opted to sync a different set of datatypes.
     GU_TRIGGER = 12;  // The client is in 'normal' mode.  It may have several
                       // reasons for requesting an update.  See the per-type
                       // GetUpdateTriggers message for more details.
     RETRY = 13;  // A retry GU to pick up updates missed by last GU due to
                  // replication delay, missing hints, etc.
   }
 }