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

sync/notifier/registration_manager.cc

Index: sync/notifier/registration_manager.cc
diff --git a/sync/notifier/registration_manager.cc b/sync/notifier/registration_manager.cc
index 3d20df0b23e2112b2f567150e25bb272a4fc23d8..04e39349b425be9c590c851eea0ab0b638ac6ad1 100644
--- a/sync/notifier/registration_manager.cc
+++ b/sync/notifier/registration_manager.cc
@@ -66,7 +66,7 @@ RegistrationManager::~RegistrationManager() {
   STLDeleteValues(&registration_statuses_);
 }
 
-void RegistrationManager::SetRegisteredIds(const ObjectIdSet& ids) {
+ObjectIdSet RegistrationManager::SetRegisteredIds(const ObjectIdSet& ids) {
   DCHECK(CalledOnValidThread());
 
   const ObjectIdSet& old_ids = GetRegisteredIds();
@@ -92,6 +92,8 @@ void RegistrationManager::SetRegisteredIds(const ObjectIdSet& ids) {
       TryRegisterId(*it, false /* is-retry */);
     }
   }
+
+  return to_unregister;
 }
 
 void RegistrationManager::MarkRegistrationLost(