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

sync/test/engine/mock_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_TEST_ENGINE_MOCK_UPDATE_HANDLER_H_
 #define SYNC_TEST_ENGINE_MOCK_UPDATE_HANDLER_H_
 
 #include "base/compiler_specific.h"
 #include "sync/engine/update_handler.h"
 #include "sync/internal_api/public/base/model_type.h"
 #include "sync/protocol/sync.pb.h"
 
 namespace syncer {
 
 class MockUpdateHandler : public UpdateHandler {
  public:
   explicit MockUpdateHandler(ModelType type);
   virtual ~MockUpdateHandler();
 
   // UpdateHandler implementation.
   virtual void GetDownloadProgress(
       sync_pb::DataTypeProgressMarker* progress_marker) const OVERRIDE;
   virtual void GetDataTypeContext(sync_pb::DataTypeContext* context) const
       OVERRIDE;
-  virtual void ProcessGetUpdatesResponse(
+  virtual SyncerError ProcessGetUpdatesResponse(
       const sync_pb::DataTypeProgressMarker& progress_marker,
       const sync_pb::DataTypeContext& mutated_context,
       const SyncEntityList& applicable_updates,
       sessions::StatusController* status) OVERRIDE;
   virtual void ApplyUpdates(sessions::StatusController* status) OVERRIDE;
   virtual void PassiveApplyUpdates(sessions::StatusController* status) OVERRIDE;
 
   // Returns the number of times ApplyUpdates() was invoked.
   int GetApplyUpdatesCount();
 
   // Returns the number of times PassiveApplyUpdates() was invoked.
   int GetPassiveApplyUpdatesCount();
 
  private:
   sync_pb::DataTypeProgressMarker progress_marker_;
 
   int apply_updates_count_;
   int passive_apply_updates_count_;
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_TEST_ENGINE_MOCK_UPDATE_HANDLER_H_