v.io/x/ref: add namespace factory option to rt.Init()

This option will be used by the android runtime to override
default namespace creation.  We need this functionality in order
to implement remote GCM wakeups, which are performed by the
special mounttable we run.

Change-Id: I6b20329a550a07c84c41c2a9412c64f860803473
diff --git a/runtime/factories/roaming/roaming.go b/runtime/factories/roaming/roaming.go
index 926e486..e8e6ea3 100644
--- a/runtime/factories/roaming/roaming.go
+++ b/runtime/factories/roaming/roaming.go
@@ -72,7 +72,7 @@
 
 	publisher := pubsub.NewPublisher()
 
-	runtime, ctx, shutdown, err := rt.Init(ctx, ac, discoveryFactory, nil, &listenSpec, publisher, commonFlags.RuntimeFlags(), reservedDispatcher, 0)
+	runtime, ctx, shutdown, err := rt.Init(ctx, ac, discoveryFactory, nil, nil, &listenSpec, publisher, commonFlags.RuntimeFlags(), reservedDispatcher, 0)
 	if err != nil {
 		ishutdown()
 		return nil, nil, nil, err