sync: Support nudges from non-blocking sync types

sync/engine/nudge_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_NUDGE_HANDLER_H_
+#define SYNC_ENGINE_NUDGE_HANDLER_H_
+
+#include "base/compiler_specific.h"
+#include "sync/base/sync_export.h"
+#include "sync/internal_api/public/base/model_type.h"
+
+namespace syncer {
+
+class SYNC_EXPORT_PRIVATE NudgeHandler {
+ public:
+  NudgeHandler();
+  virtual ~NudgeHandler();
+
+  virtual void NudgeForInitialDownload(syncer::ModelType type) = 0;
+  virtual void NudgeForCommit(syncer::ModelType type) = 0;
+  virtual void NudgeForRefresh(syncer::ModelType type) = 0;
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_ENGINE_NUDGE_HANDLER_H_