Update sync API to support attachments.

sync/api/sync_change_processor.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_API_SYNC_CHANGE_PROCESSOR_H_
 #define SYNC_API_SYNC_CHANGE_PROCESSOR_H_
 
 #include <vector>
 
 #include "sync/api/sync_data.h"
 #include "sync/api/sync_error.h"
 #include "sync/base/sync_export.h"
 #include "sync/internal_api/public/base/model_type.h"
 
 namespace tracked_objects {
 class Location;
 }  // namespace tracked_objects
 
 namespace syncer {
 
 class SyncChange;
 
 typedef std::vector<SyncChange> SyncChangeList;
 
 // An interface for services that handle receiving SyncChanges.
 class SYNC_EXPORT SyncChangeProcessor {
  public:
+  typedef base::Callback<void(const SyncData&)> GetSyncDataCallback;
+
   SyncChangeProcessor();
   virtual ~SyncChangeProcessor();
 
   // Process a list of SyncChanges.
   // Returns: A default SyncError (IsSet() == false) if no errors were
   //          encountered, and a filled SyncError (IsSet() == true)
   //          otherwise.
   // Inputs:
   //   |from_here|: allows tracking of where sync changes originate.
   //   |change_list|: is the list of sync changes in need of processing.
   virtual SyncError ProcessSyncChanges(
       const tracked_objects::Location& from_here,
       const SyncChangeList& change_list) = 0;
 
   // Fills a list of SyncData. This should create an up to date representation
   // of all the data known to the ChangeProcessor for |datatype|, and
   // should match/be a subset of the server's view of that datatype.
   //
   // WARNING: This can be a potentially slow & memory intensive operation and
   // should only be used when absolutely necessary / sparingly.
   virtual SyncDataList GetAllSyncData(ModelType type) const = 0;
+
+  // Retrieves the SyncData identified by |type| and |sync_tag| and invokes
+  // |callback| asynchronously. If no such SyncData exists locally, IsValid on
+  // the SyncData passed to |callback| will return false.
+  //
+  // This is an asynchronous local operation that may result in disk IO.
+  //
+  // Refer to sync_data.h for a description of |sync_tag|.
+  //
+  // TODO:(maniscalco): N.B. this method should really be pure virtual. An
+  // implentation is provided here just to verify that everything compiles.
+  // Update this method to be pure virtual (bug 353300).
+  virtual void GetSyncData(const ModelType& type,
+                           const std::string& sync_tag,
+                           const GetSyncDataCallback& callback) const {}
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_SYNC_CHANGE_PROCESSOR_H_