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

sync/tools/sync_listen_notifications.cc

Index: sync/tools/sync_listen_notifications.cc
diff --git a/sync/tools/sync_listen_notifications.cc b/sync/tools/sync_listen_notifications.cc
index 3f3955c372fdd18c608252e29357aabe0d38f371..24789fd76cf8f73d55937f710fb2769178fd02f2 100644
--- a/sync/tools/sync_listen_notifications.cc
+++ b/sync/tools/sync_listen_notifications.cc
@@ -100,6 +100,12 @@ class NullInvalidationStateTracker
               << ObjectIdToString(id) << " to " << max_invalidation_version;
   }
 
+  virtual void Forget(const ObjectIdSet& ids) OVERRIDE {
+    for (ObjectIdSet::const_iterator it = ids.begin(); it != ids.end(); ++it) {
+      LOG(INFO) << "Forgetting saved state for " << ObjectIdToString(*it);
+    }
+  }
+
   virtual std::string GetInvalidationState() const OVERRIDE {
     return std::string();
   }