Add code for new invalidation local ack system

sync/internal_api/public/base/invalidation.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SYNC_INTERNAL_API_PUBLIC_BASE_INVALIDATION_H_
 #define SYNC_INTERNAL_API_PUBLIC_BASE_INVALIDATION_H_
 
 #include <string>
 
 #include "base/basictypes.h"
@@ skipping 36 matching lines @@
   int64 version() const;
 
   // Safe to call only if is_unknown_version() returns false.
   const std::string& payload() const;
 
   const AckHandle& ack_handle() const;
 
   // TODO(rlarocque): Remove this method and use AckHandlers instead.
   void set_ack_handle(const AckHandle& ack_handle);
 
+  // Functions from the alternative ack tracking framework.
+  // Currently unused.
   void set_ack_handler(syncer::WeakHandle<AckHandler> ack_handler);
+  bool SupportsAcknowledgement() const;
+  void Acknowledge() const;
 
-  // True if this class has a valid AckHandler.
-  bool SupportsAcknowledgement() const;
-
-  // TODO(rlarocque): Re-enable these when we switch to AckHandlers.
-  // void Acknowledge() const;
-  // void Drop(DroppedInvalidationTracker* tracker) const;
+  // Drops an invalidation.
+  //
+  // The drop record will be tracked by the specified
+  // DroppedInvalidationTracker.  The caller should hang on to this tracker.  It
+  // will need to use it when it recovers from this drop event.  See the
+  // documentation of DroppedInvalidationTracker for more details.
+  void Drop(DroppedInvalidationTracker* tracker) const;
 
   scoped_ptr<base::DictionaryValue> ToValue() const;
   std::string ToString() const;
 
  private:
   Invalidation(const invalidation::ObjectId& id,
                bool is_unknown_version,
                int64 version,
                const std::string& payload,
                AckHandle ack_handle);
@@ skipping 13 matching lines @@
   std::string payload_;
 
   // A locally generated unique ID used to manage local acknowledgements.
   AckHandle ack_handle_;
   syncer::WeakHandle<AckHandler> ack_handler_;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_BASE_INVALIDATION_H_