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

sync/syncable/directory_backing_store.cc

Index: sync/syncable/directory_backing_store.cc
diff --git a/sync/syncable/directory_backing_store.cc b/sync/syncable/directory_backing_store.cc
index e466548b172ee9c039af93aa1d51f837bb63a017..ec28a53d661a47fcc9baaa99774301c01d953701 100644
--- a/sync/syncable/directory_backing_store.cc
+++ b/sync/syncable/directory_backing_store.cc
@@ -70,6 +70,11 @@ void BindFields(const EntryKernel& entry,
     entry.ref(static_cast<UniquePositionField>(i)).SerializeToString(&temp);
     statement->BindBlob(index++, temp.data(), temp.length());
   }
+  for (; i < ATTACHMENT_METADATA_FIELDS_END; ++i) {
+    std::string temp;
+    entry.ref(static_cast<AttachmentMetadataField>(i)).SerializeToString(&temp);
+    statement->BindBlob(index++, temp.data(), temp.length());
+  }
 }
 
 // The caller owns the returned EntryKernel*.  Assumes the statement currently
@@ -114,6 +119,10 @@ scoped_ptr<EntryKernel> UnpackEntry(sql::Statement* statement) {
     kernel->mutable_ref(static_cast<UniquePositionField>(i)) =
         UniquePosition::FromProto(proto);
   }
+  for (; i < ATTACHMENT_METADATA_FIELDS_END; ++i) {
+    kernel->mutable_ref(static_cast<AttachmentMetadataField>(i)).ParseFromArray(
+        statement->ColumnBlob(i), statement->ColumnByteLength(i));
+  }
   return kernel.Pass();
 }