Keep track of which attachments are referenced by which sync entries.

sync/sync_internal_api.gypi

 # Copyright 2013 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.
 
 {
   'include_dirs': [
     '..',
   ],
   'defines': [
     'SYNC_IMPLEMENTATION',
@@ skipping 27 matching lines @@
     'internal_api/js_mutation_event_observer.h',
     'internal_api/js_sync_encryption_handler_observer.cc',
     'internal_api/js_sync_encryption_handler_observer.h',
     'internal_api/js_sync_manager_observer.cc',
     'internal_api/js_sync_manager_observer.h',
     'internal_api/non_blocking_type_processor.cc',
     'internal_api/protocol_event_buffer.cc',
     'internal_api/protocol_event_buffer.h',
     'internal_api/public/base/ack_handle.cc',
     'internal_api/public/base/ack_handle.h',
+    'internal_api/public/base/attachment_id_proto.cc',
+    'internal_api/public/base/attachment_id_proto.h',
     'internal_api/public/base/cancelation_observer.cc',
     'internal_api/public/base/cancelation_observer.h',
     'internal_api/public/base/cancelation_signal.cc',
     'internal_api/public/base/cancelation_signal.h',
     'internal_api/public/base/enum_set.h',
     'internal_api/public/base/enum_set.h',
     'internal_api/public/base/invalidation.cc',
     'internal_api/public/base/invalidation.h',
     'internal_api/public/base/model_type.h',
     'internal_api/public/base/node_ordinal.cc',
@@ skipping 91 matching lines @@
     'internal_api/sync_rollback_manager_base.h',
     'internal_api/syncapi_internal.cc',
     'internal_api/syncapi_internal.h',
     'internal_api/syncapi_server_connection_manager.cc',
     'internal_api/syncapi_server_connection_manager.h',
     'internal_api/user_share.cc',
     'internal_api/write_node.cc',
     'internal_api/write_transaction.cc',
   ],
 }