Checkpointing some Sync Attachment work. Added SyncAttachment and SyncAttachmentId.

sync/protocol/sync.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";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
 
 import "app_list_specifics.proto";
 import "app_notification_specifics.proto";
 import "app_setting_specifics.proto";
 import "app_specifics.proto";
 import "article_specifics.proto";
+import "attachments.proto";
 import "autofill_specifics.proto";
 import "bookmark_specifics.proto";
 import "client_commands.proto";
 import "client_debug_info.proto";
 import "device_info_specifics.proto";
 import "dictionary_specifics.proto";
 import "encryption.proto";
 import "experiments_specifics.proto";
 import "extension_setting_specifics.proto";
 import "extension_specifics.proto";
@@ skipping 853 matching lines @@
     // is absent then the whole client (all datatypes) is throttled.
     repeated int32 error_data_type_ids = 5;
   }
   optional Error error = 13;
 
   // The new per-client state for this client. If set, should be persisted and
   // sent with any subsequent ClientToServerMessages.
   optional ChipBag new_bag_of_chips = 14;
 };