Implement new invalidations ack tracking system

sync/notifier/unacked_invalidation_set.h

Index: sync/notifier/unacked_invalidation_set.h
diff --git a/sync/notifier/unacked_invalidation_set.h b/sync/notifier/unacked_invalidation_set.h
index 2824e6a5a3b3a7818b42816deac3a0615be3fb03..aae9cdab3ebcd2008f9e9b9374a440d7e7db53bf 100644
--- a/sync/notifier/unacked_invalidation_set.h
+++ b/sync/notifier/unacked_invalidation_set.h
@@ -18,6 +18,10 @@ class DictionaryValue;
 
 namespace syncer {
 
+namespace test_util {
+class UnackedInvalidationSetEqMatcher;
+}  // test_util
+
 class SingleObjectInvalidationSet;
 class ObjectIdInvalidationMap;
 class AckHandle;
@@ -88,7 +92,7 @@ class SYNC_EXPORT UnackedInvalidationSet {
 
  private:
   // Allow this test helper to have access to our internals.
-  friend class UnackedInvalidationSetEqMatcher;
+  friend class test_util::UnackedInvalidationSetEqMatcher;
 
   typedef std::set<Invalidation, InvalidationVersionLessThan> InvalidationsSet;