Client-side changes to support retry GU.

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 123 matching lines @@
                                     // support for a new datatype was recently
                                     // released via a software auto-update.
     MIGRATION = 8;         // The client is in configuration mode because a
                            // MIGRATION_DONE error previously returned by the
                            // server necessitated resynchronization.
     NEW_CLIENT = 9;        // The client is in configuration mode because the
                            // user enabled sync for the first time.  Not to be
                            // 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.
+    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.
   }
 }