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

sync/syncable/mutable_entry.cc

Index: sync/syncable/mutable_entry.cc
diff --git a/sync/syncable/mutable_entry.cc b/sync/syncable/mutable_entry.cc
index 863e65b8b32f8424e183227ebe24e5f604ac4149..d9d5daa890f0c13fedd1723bfe9b27794a4b2d14 100644
--- a/sync/syncable/mutable_entry.cc
+++ b/sync/syncable/mutable_entry.cc
@@ -233,6 +233,20 @@ bool MutableEntry::PutPredecessor(const Id& predecessor_id) {
   return true;
 }
 
+void MutableEntry::PutAttachmentMetadata(
+    const sync_pb::AttachmentMetadata& attachment_metadata) {
+  DCHECK(kernel_);
+  write_transaction()->TrackChangesTo(kernel_);
+  if (kernel_->ref(ATTACHMENT_METADATA).SerializeAsString() !=
+      attachment_metadata.SerializeAsString()) {
+    dir()->UpdateAttachmentIndex(GetMetahandle(),
+                                 kernel_->ref(ATTACHMENT_METADATA),
+                                 attachment_metadata);
+    kernel_->put(ATTACHMENT_METADATA, attachment_metadata);
+    kernel_->mark_dirty(&dir()->kernel_->dirty_metahandles);
+  }
+}
+
 // This function sets only the flags needed to get this entry to sync.
 bool MarkForSyncing(MutableEntry* e) {
   DCHECK_NE(static_cast<MutableEntry*>(NULL), e);