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

sync/syncable/mutable_entry.h

Index: sync/syncable/mutable_entry.h
diff --git a/sync/syncable/mutable_entry.h b/sync/syncable/mutable_entry.h
index 8c2f2ab5492bcbfe4f785593f5441e5ccbe1c272..c064358082ec274331cc2bde4fcb36c6de12a7b5 100644
--- a/sync/syncable/mutable_entry.h
+++ b/sync/syncable/mutable_entry.h
@@ -59,6 +59,9 @@ class SYNC_EXPORT_PRIVATE MutableEntry : public ModelNeutralMutableEntry {
   // ID to put the node in first position.
   bool PutPredecessor(const Id& predecessor_id);
 
+  void PutAttachmentMetadata(
+      const sync_pb::AttachmentMetadata& attachment_metadata);
+
  private:
   // Kind of redundant. We should reduce the number of pointers
   // floating around if at all possible. Could we store this in Directory?