Add AttachmentMetadata to Sync's EntryKernel.

sync/syncable/entry.h

Index: sync/syncable/entry.h
diff --git a/sync/syncable/entry.h b/sync/syncable/entry.h
index 09ff9c7004f252e839734c84d96747417d254293..47b9c593301d0d96f88aefef8f4f9f7d5d80a6b1 100644
--- a/sync/syncable/entry.h
+++ b/sync/syncable/entry.h
@@ -196,6 +196,11 @@ class SYNC_EXPORT Entry {
     return kernel_->ref(UNIQUE_POSITION);
   }
 
+  const sync_pb::AttachmentMetadata& GetAttachmentMetadata() const {
+    DCHECK(kernel_);
+    return kernel_->ref(ATTACHMENT_METADATA);
+  }
+
   bool GetSyncing() const {
     DCHECK(kernel_);
     return kernel_->ref(SYNCING);