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

sync/notifier/registration_manager.h

Index: sync/notifier/registration_manager.h
diff --git a/sync/notifier/registration_manager.h b/sync/notifier/registration_manager.h
index 20c753b934dd3b969dadf4d83aa493559e21a191..28922ef94b15a81ca7459357ea3d872bb9ae8b24 100644
--- a/sync/notifier/registration_manager.h
+++ b/sync/notifier/registration_manager.h
@@ -67,9 +67,10 @@ class RegistrationManager : public base::NonThreadSafe {
 
   virtual ~RegistrationManager();
 
-  // Registers all object IDs included in the given set (that are not
-  // already disabled) and unregisters all other object IDs.
-  void SetRegisteredIds(const ObjectIdSet& ids);
+  // Registers all object IDs included in the given set (that are not already
+  // disabled) and unregisters all other object IDs. The return value is the set
+  // of object IDs that was unregistered.
+  ObjectIdSet SetRegisteredIds(const ObjectIdSet& ids);
 
   // Marks the registration for the |id| lost and re-registers
   // it (unless it's disabled).