[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 80b97b892741fedfa11df17a0c645389d27a5aa9..1b01d5d56563eecabf681ccdef34ba9e5b16819f 100644
--- a/sync/notifier/sync_notifier.h
+++ b/sync/notifier/sync_notifier.h
@@ -22,10 +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