Implement new invalidations ack tracking system

sync/internal_api/public/base/invalidation.cc

Index: sync/internal_api/public/base/invalidation.cc
diff --git a/sync/internal_api/public/base/invalidation.cc b/sync/internal_api/public/base/invalidation.cc
index 1292a51bbe670198f22c896ad653bd760e19e1f8..ff7a5a78fd4dc542856a768b08565552e1823150 100644
--- a/sync/internal_api/public/base/invalidation.cc
+++ b/sync/internal_api/public/base/invalidation.cc
@@ -111,10 +111,6 @@ const AckHandle& Invalidation::ack_handle() const {
   return ack_handle_;
 }
 
-void Invalidation::set_ack_handle(const AckHandle& ack_handle) {
-  ack_handle_ = ack_handle;
-}
-
 void Invalidation::set_ack_handler(syncer::WeakHandle<AckHandler> handler) {
   ack_handler_ = handler;

[tim (not reviewing), 2013/11/20 18:27:04]

At the very least, this shouldn't ever change from a non-null value to a
different non-null value, correct?  A dcheck would be a good idea.  Might
consider renaming to InitAckHandler(...), but you don't have to.. up to you.

[rlarocque, 2013/11/21 20:09:27]

Done.

 }
@@ -135,10 +131,12 @@ void Invalidation::Acknowledge() const {
 void Invalidation::Drop(DroppedInvalidationTracker* tracker) const {
   DCHECK(tracker->object_id() == object_id());
   tracker->RecordDropEvent(ack_handler_, ack_handle_);
-  ack_handler_.Call(FROM_HERE,
-                    &AckHandler::Drop,
-                    id_,
-                    ack_handle_);
+  if (SupportsAcknowledgement()) {
+    ack_handler_.Call(FROM_HERE,
+                      &AckHandler::Drop,
+                      id_,
+                      ack_handle_);
+  }
 }
 
 bool Invalidation::Equals(const Invalidation& other) const {