discovery: Add timestamp to discovery.Update.

The syncbase discovery service needs to be able to order updates from
global discovery and neighborhood discovery based on timestamps in the
case of a changed advertisement.
This change exposes timestamps from discovery.Update.

MultiPart: 2/5
Change-Id: Ib109b2f6371acf17aab9c922fb4774f983bd7776
diff --git a/impl/google/discovery/util.go b/impl/google/discovery/util.go
index fc2dd1d..0457db0 100644
--- a/impl/google/discovery/util.go
+++ b/impl/google/discovery/util.go
@@ -38,7 +38,8 @@
 		return jutil.NullObject, err
 	}
 	ref := jutil.GoNewRef(&update) // Un-refed when jUpdate is finalized.
-	jUpdate, err := jutil.NewObject(env, jUpdateImplClass, []jutil.Sign{jutil.LongSign, jutil.BoolSign, advertisementSign}, int64(ref), update.IsLost(), jAd)
+	jUpdate, err := jutil.NewObject(env, jUpdateImplClass, []jutil.Sign{jutil.LongSign, jutil.BoolSign, advertisementSign, jutil.LongSign},
+		int64(ref), update.IsLost(), jAd, update.Timestamp().UnixNano())
 	if err != nil {
 		jutil.GoDecRef(ref)
 		return jutil.NullObject, err