sync: Add GetAllSyncData to sync/api

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:
   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;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_SYNC_CHANGE_PROCESSOR_H_