Revert 150990 - [Sync] Avoid unregistering object IDs on shutdown

sync/notifier/sync_notifier.h

Index: sync/notifier/sync_notifier.h
===================================================================
--- sync/notifier/sync_notifier.h	(revision 150991)
+++ sync/notifier/sync_notifier.h	(working copy)
@@ -22,46 +22,12 @@
   SyncNotifier() {}
   virtual ~SyncNotifier() {}
 
-  // Clients should follow the pattern below:
-  //
-  // When starting the client:
-  //
-  //   notifier->RegisterHandler(client_handler);
-  //
-  // When the set of IDs to register changes for the client during its lifetime
-  // (i.e., between calls to RegisterHandler(client_handler) and
-  // UnregisterHandler(client_handler):
-  //
-  //   notifier->UpdateRegisteredIds(client_handler, client_ids);
-  //
-  // When shutting down the client for browser shutdown:
-  //
-  //   notifier->UnregisterHandler(client_handler);
-  //
-  // Note that there's no call to UpdateRegisteredIds() -- this is because the
-  // invalidation API persists registrations across browser restarts.
-  //
-  // When permanently shutting down the client, e.g. when disabling the related
-  // feature:
-  //
-  //   notifier->UpdateRegisteredIds(client_handler, ObjectIdSet());
-  //   notifier->UnregisterHandler(client_handler);
-
-  // Starts sending notifications to |handler|.  |handler| must not be NULL,
-  // and it must already be registered.
-  virtual void RegisterHandler(SyncNotifierObserver* handler) = 0;
-
-  // Updates the set of ObjectIds associated with |handler|.  |handler| must
-  // not be NULL, and must already be registered.  An ID must be registered for
-  // at most one handler.
+  // 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,
                                    const ObjectIdSet& ids) = 0;
 
-  // Stops sending notifications to |handler|.  |handler| must not be NULL, and
-  // it must already be registered.  Note that this doesn't unregister the IDs
-  // associated with |handler|.
-  virtual void UnregisterHandler(SyncNotifierObserver* handler) = 0;
-
   // SetUniqueId must be called once, before any call to
   // UpdateCredentials.  |unique_id| should be a non-empty globally
   // unique string.