[Sync] Avoid unregistering object IDs on shutdown

sync/notifier/sync_notifier.h

Index: sync/notifier/sync_notifier.h
diff --git a/sync/notifier/sync_notifier.h b/sync/notifier/sync_notifier.h
index 48bc1f10b44630cda2be44f39fbb6b11ce1f8f34..0730f8a9ed1711f5f90d42a848018fae63d05b9d 100644
--- a/sync/notifier/sync_notifier.h
+++ b/sync/notifier/sync_notifier.h
@@ -22,11 +22,16 @@ class SyncNotifier {
   SyncNotifier() {}
   virtual ~SyncNotifier() {}
 
-  // Updates the set of ObjectIds associated with a given
-  // |handler|. Passing an empty ObjectIdSet will unregister
-  // |handler|. There should be at most one handler registered per
-  // object id.
-  virtual void UpdateRegisteredIds(SyncNotifierObserver* handler,
+  // Sets the handler for the given name.  Pass in NULL for |handler|
+  // if you want to remove the handler for the given name.  (This
+  // doesn't unregister the IDs for the given name, though.)  A
+  // handler must be set for at most one name.
+  virtual void SetHandler(const std::string& handler_name,
+                          SyncNotifierObserver* handler) = 0;
+
+  // Updates the set of ObjectIds associated with a given handler (via
+  // its name). An ID must be registered for at most one handler.
+  virtual void UpdateRegisteredIds(const std::string& handler_name,
                                    const ObjectIdSet& ids) = 0;
 
   // SetUniqueId must be called once, before any call to