[Sync] Add sync api support for context changes

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"
@@ skipping 45 matching lines @@
   // 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 {}
+
+  // Updates the context for |type|. Default implementation does nothing.
+  // A type's context is a per-client blob that can affect all SyncData
+  // sent to/from the server, much like a cookie.
+  virtual syncer::SyncError UpdateDataTypeContext(ModelType type,
+                                                  const std::string& context);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_SYNC_CHANGE_PROCESSOR_H_