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

sync/syncable/entry_kernel.h

Index: sync/syncable/entry_kernel.h
diff --git a/sync/syncable/entry_kernel.h b/sync/syncable/entry_kernel.h
index b6bc2ab34c446a0035eed56187f1893b2248865b..63804206a220e3c5f62c50a14119eb36c59e2beb 100644
--- a/sync/syncable/entry_kernel.h
+++ b/sync/syncable/entry_kernel.h
@@ -333,6 +333,10 @@ struct SYNC_EXPORT_PRIVATE EntryKernel {
   inline UniquePosition& mutable_ref(UniquePositionField field) {
     return unique_position_fields[field - UNIQUE_POSITION_FIELDS_BEGIN];
   }
+  inline sync_pb::AttachmentMetadata& mutable_ref(
+      AttachmentMetadataField field) {
+    return attachment_metadata_fields[field - ATTACHMENT_METADATA_FIELDS_BEGIN];
+  }
 
   ModelType GetModelType() const;
   ModelType GetServerModelType() const;