Add InvalidationStateTracker::Forget() to erase an entry from storage.

sync/notifier/registration_manager.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // A class that manages the registration of types for server-issued
 // notifications.
 
 #ifndef SYNC_NOTIFIER_REGISTRATION_MANAGER_H_
 #define SYNC_NOTIFIER_REGISTRATION_MANAGER_H_
 
@@ skipping 50 matching lines @@
                    ObjectIdLessThan>
       PendingRegistrationMap;
 
   // Does not take ownership of |invalidation_client_|.
   explicit RegistrationManager(
       invalidation::InvalidationClient* invalidation_client);
 
   virtual ~RegistrationManager();
 
   // Registers all object IDs included in the given set (that are not
-  // already disabled) and unregisters all other object IDs.
-  void UpdateRegisteredIds(const ObjectIdSet& ids);
+  // already disabled) and unregisters all other object IDs. The return value is
+  // the set of IDs that was unregistered.
+  ObjectIdSet UpdateRegisteredIds(const ObjectIdSet& ids);
 
   // Marks the registration for the |id| lost and re-registers
   // it (unless it's disabled).
   void MarkRegistrationLost(const invalidation::ObjectId& id);
 
   // Marks registrations lost for all enabled object IDs and re-registers them.
   void MarkAllRegistrationsLost();
 
   // Marks the registration for the |id| permanently lost and blocks any future
   // registration attempts.
@@ skipping 93 matching lines @@
   RegistrationStatusMap registration_statuses_;
   // Weak pointer.
   invalidation::InvalidationClient* invalidation_client_;
 
   DISALLOW_COPY_AND_ASSIGN(RegistrationManager);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_NOTIFIER_REGISTRATION_MANAGER_H_