Add server_attachment_metadata field to EntryKernel and sync database.

sync/syncable/entry.h

Index: sync/syncable/entry.h
diff --git a/sync/syncable/entry.h b/sync/syncable/entry.h
index 5faa6e8f65917474ac063f3cddbb75ab8324b889..184377a3766333d5121a700481d06c78c6863b3e 100644
--- a/sync/syncable/entry.h
+++ b/sync/syncable/entry.h
@@ -210,6 +210,11 @@ class SYNC_EXPORT Entry {
     return kernel_->ref(ATTACHMENT_METADATA);
   }
 
+  const sync_pb::AttachmentMetadata& GetServerAttachmentMetadata() const {
+    DCHECK(kernel_);
+    return kernel_->ref(SERVER_ATTACHMENT_METADATA);
+  }
+
   bool GetSyncing() const {
     DCHECK(kernel_);
     return kernel_->ref(SYNCING);