Client-side changes to support retry GU.

sync/protocol/get_updates_caller_info.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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
@@ skipping 24 matching lines @@
                            // 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.
     DATATYPE_REFRESH = 11;    // A datatype has requested a refresh. This is
                               // typically used when datatype's have custom
                               // sync UI, e.g. sessions.
+    RETRY = 13;  // A follow-up GU to pick up updates missed by previous GU.
   }
 
   required GetUpdatesSource source = 1;
 
   // True only if notifications were enabled for this GetUpdateMessage.
   optional bool notifications_enabled = 2;
 };