[Sync] Avoid unregistering object IDs on shutdown

sync/notifier/p2p_notifier.cc

Index: sync/notifier/p2p_notifier.cc
diff --git a/sync/notifier/p2p_notifier.cc b/sync/notifier/p2p_notifier.cc
index 5d5aeffc04bd33bdd50a1251e96620ff928d4794..307bc3002da16dec418695d0bbd80d718a2b5973 100644
--- a/sync/notifier/p2p_notifier.cc
+++ b/sync/notifier/p2p_notifier.cc
@@ -157,10 +157,20 @@ P2PNotifier::~P2PNotifier() {
   push_client_->RemoveObserver(this);
 }
 
-void P2PNotifier::UpdateRegisteredIds(SyncNotifierObserver* handler,
+void P2PNotifier::SetHandler(const std::string& handler_name,
+                             SyncNotifierObserver* handler) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  helper_.SetHandler(handler_name, handler);
+}
+
+void P2PNotifier::UpdateRegisteredIds(const std::string& handler_name,
                                       const ObjectIdSet& ids) {
-  const ModelTypeSet enabled_types = ObjectIdSetToModelTypeSet(
-      helper_.UpdateRegisteredIds(handler, ids));
+  // TODO(akalin): Handle arbitrary object IDs
+  // (http://crbug.com/140411).
+  DCHECK(thread_checker_.CalledOnValidThread());
+  helper_.UpdateRegisteredIds(handler_name, ids);
+  const ModelTypeSet enabled_types =
+      ObjectIdSetToModelTypeSet(helper_.GetAllRegisteredIds());
   const ModelTypeSet new_enabled_types =
       Difference(enabled_types, enabled_types_);
   const P2PNotificationData notification_data(