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

sync/notifier/invalidation_state_tracker.h

Index: sync/notifier/invalidation_state_tracker.h
diff --git a/sync/notifier/invalidation_state_tracker.h b/sync/notifier/invalidation_state_tracker.h
index 8f080fade20c80891d0a7431dcc123c9b1c28110..2d67f541bd17d508403a6c5a5a1b2fa32c97674a 100644
--- a/sync/notifier/invalidation_state_tracker.h
+++ b/sync/notifier/invalidation_state_tracker.h
@@ -29,6 +29,8 @@ class InvalidationStateTracker {
   // version for |model_type|.
   virtual void SetMaxVersion(const invalidation::ObjectId& id,
                              int64 max_version) = 0;
+  // Removes all state tracked for |id|.
+  virtual void Forget(const invalidation::ObjectId& id) = 0;
 
   // Used by InvalidationClient for persistence. |state| is opaque data we can
   // present back to the client (e.g. after a restart) for it to bootstrap