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

sync/internal_api/write_node.cc

Index: sync/internal_api/write_node.cc
diff --git a/sync/internal_api/write_node.cc b/sync/internal_api/write_node.cc
index 6a634eb85c970a93f3b5d00400a7e5d267d2dfe0..c3ef0815a86213ecffe1e9df6034de602bc89471 100644
--- a/sync/internal_api/write_node.cc
+++ b/sync/internal_api/write_node.cc
@@ -464,6 +464,11 @@ bool WriteNode::SetPosition(const BaseNode& new_parent,
   return PutPredecessor(predecessor);
 }
 
+void WriteNode::SetAttachmentMetadata(
+    const sync_pb::AttachmentMetadata& attachment_metadata) {
+  entry_->PutAttachmentMetadata(attachment_metadata);
+}
+
 const syncable::Entry* WriteNode::GetEntry() const {
   return entry_;
 }