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 1c941402be82402eb2ed05c21a0d76d1a6f026d8..4c59e012f8b8026c704e0ad0f4ce1c7ca67fbacb 100644
--- a/sync/notifier/registration_manager.cc
+++ b/sync/notifier/registration_manager.cc
@@ -66,7 +66,7 @@ RegistrationManager::~RegistrationManager() {
   STLDeleteValues(&registration_statuses_);
 }
 
-void RegistrationManager::UpdateRegisteredIds(const ObjectIdSet& ids) {
+ObjectIdSet RegistrationManager::UpdateRegisteredIds(const ObjectIdSet& ids) {
   DCHECK(CalledOnValidThread());
 
   const ObjectIdSet& old_ids = GetRegisteredIds();
@@ -92,6 +92,8 @@ void RegistrationManager::UpdateRegisteredIds(const ObjectIdSet& ids) {
       TryRegisterId(*it, false /* is-retry */);
     }
   }
+
+  return to_unregister;
 }
 
 void RegistrationManager::MarkRegistrationLost(