[Sync] Add support for retrying a getupdates due to a context change

sync/engine/update_handler.h

 // Copyright 2014 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_ENGINE_UPDATE_HANDLER_H_
 #define SYNC_ENGINE_UPDATE_HANDLER_H_
 
 #include <vector>
 
 #include "sync/base/sync_export.h"
@@ skipping 31 matching lines @@
 
   // Processes the contents of a GetUpdates response message.
   //
   // Should be invoked with the progress marker and set of SyncEntities from a
   // single GetUpdates response message.  The progress marker's type must match
   // this update handler's type, and the set of SyncEntities must include all
   // entities of this type found in the response message.
   //
   // In this context, "applicable_updates" means the set of updates belonging to
   // this type.
-  virtual void ProcessGetUpdatesResponse(
+  //
+  // Returns true if the response was successfully handled, false if an error
+  // occurred and the GetUpdates should be retried.
+  virtual bool ProcessGetUpdatesResponse(
       const sync_pb::DataTypeProgressMarker& progress_marker,
       const sync_pb::DataTypeContext& mutated_context,
       const SyncEntityList& applicable_updates,
       sessions::StatusController* status) = 0;
 
   // Called at the end of a non-configure GetUpdates loop to apply any unapplied
   // updates.
   virtual void ApplyUpdates(sessions::StatusController* status) = 0;
 
   // Called at the end of a configure GetUpdates loop to perform any required
   // post-initial-download update application.
   virtual void PassiveApplyUpdates(sessions::StatusController* status) = 0;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_ENGINE_UPDATE_HANDLER_H_