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

sync/internal_api/public/base_transaction.h

Index: sync/internal_api/public/base_transaction.h
diff --git a/sync/internal_api/public/base_transaction.h b/sync/internal_api/public/base_transaction.h
index 2f4fa3c91b103ebf24a8c020dfdb05f7c76e1b21..c75086e526f6e3fb2d1b12c248fec925c9638b34 100644
--- a/sync/internal_api/public/base_transaction.h
+++ b/sync/internal_api/public/base_transaction.h
@@ -24,6 +24,9 @@ class Directory;
 // syncable, and are used in a similar way. Unlike syncable::BaseTransaction,
 // whose construction requires an explicit syncable::Directory, a sync
 // API BaseTransaction is created from a UserShare object.
+//
+// Note, these transactions are not atomic. Individual operations can
+// fail. There is no built-in rollback or undo mechanism.
 class SYNC_EXPORT BaseTransaction {
  public:
   // Provide access to the underlying syncable objects from BaseNode.