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

sync/notifier/registration_manager_unittest.cc

Index: sync/notifier/registration_manager_unittest.cc
diff --git a/sync/notifier/registration_manager_unittest.cc b/sync/notifier/registration_manager_unittest.cc
index e2b9b91bd5a6f8e884f790fcd2eff14312cc786c..df35d71eed93797d82488afe9de9dcf9a6d4df1c 100644
--- a/sync/notifier/registration_manager_unittest.cc
+++ b/sync/notifier/registration_manager_unittest.cc
@@ -238,13 +238,19 @@ TEST_F(RegistrationManagerTest, SetRegisteredIds) {
   EXPECT_TRUE(fake_registration_manager_.GetRegisteredIdsForTest().empty());
   EXPECT_TRUE(fake_invalidation_client_.GetRegisteredIdsForTest().empty());
 
-  fake_registration_manager_.SetRegisteredIds(ids);
+  ObjectIdSet expected_unregistered_ids;
+
+  ObjectIdSet unregistered_ids =
+      fake_registration_manager_.SetRegisteredIds(ids);
+  EXPECT_EQ(expected_unregistered_ids, unregistered_ids);
   EXPECT_EQ(ids, fake_registration_manager_.GetRegisteredIdsForTest());
   EXPECT_EQ(ids, fake_invalidation_client_.GetRegisteredIdsForTest());
 
   ids.insert(GetIdForIndex(kObjectIdsCount - 1));
   ids.erase(GetIdForIndex(kObjectIdsCount - 2));
-  fake_registration_manager_.SetRegisteredIds(ids);
+  unregistered_ids = fake_registration_manager_.SetRegisteredIds(ids);
+  expected_unregistered_ids.insert(GetIdForIndex(kObjectIdsCount - 2));
+  EXPECT_EQ(expected_unregistered_ids, unregistered_ids);
   EXPECT_EQ(ids, fake_registration_manager_.GetRegisteredIdsForTest());
   EXPECT_EQ(ids, fake_invalidation_client_.GetRegisteredIdsForTest());
 }