Implement new invalidations ack tracking system

sync/notifier/p2p_invalidator.cc

Index: sync/notifier/p2p_invalidator.cc
diff --git a/sync/notifier/p2p_invalidator.cc b/sync/notifier/p2p_invalidator.cc
index 3d47f418bbc536f80021521974c2c4dbb3ea20fb..cd82e6145e8d98af3c29c33db00441c378d326ec 100644
--- a/sync/notifier/p2p_invalidator.cc
+++ b/sync/notifier/p2p_invalidator.cc
@@ -180,12 +180,6 @@ void P2PInvalidator::UnregisterHandler(InvalidationHandler* handler) {
   registrar_.UnregisterHandler(handler);
 }
 
-void P2PInvalidator::Acknowledge(const invalidation::ObjectId& id,
-                                 const AckHandle& ack_handle) {
-  DCHECK(thread_checker_.CalledOnValidThread());
-  // Do nothing for the P2P implementation.
-}
-
 InvalidatorState P2PInvalidator::GetInvalidatorState() const {
   DCHECK(thread_checker_.CalledOnValidThread());
   return registrar_.GetInvalidatorState();