[Sync] Make ModelSafeWorker a true interface

chrome/browser/sync/engine/passive_model_worker.cc

+// Copyright (c) 2011 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.
+
+#include "chrome/browser/sync/engine/passive_model_worker.h"
+
+#include "base/message_loop.h"
+
+namespace browser_sync {
+
+PassiveModelWorker::PassiveModelWorker(const MessageLoop* sync_loop)
+    : sync_loop_(sync_loop) {}
+
+PassiveModelWorker::~PassiveModelWorker() {
+}
+
+UnrecoverableErrorInfo PassiveModelWorker::DoWorkAndWaitUntilDone(
+    const WorkCallback& work) {
+  DCHECK_EQ(MessageLoop::current(), sync_loop_);
+  // Simply do the work on the current thread.
+  return work.Run();
+}
+
+ModelSafeGroup PassiveModelWorker::GetModelSafeGroup() {
+  return GROUP_PASSIVE;
+}
+
+}  // namespace browser_sync