[Sync] Move thread checking into the ModelSafeWorker interface.

components/sync/driver/glue/ui_model_worker.cc

 // Copyright (c) 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.
 
 #include "components/sync/driver/glue/ui_model_worker.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 44 matching lines @@
   }
   work_done_or_abandoned.Wait();
 
   return error_info;
 }
 
 ModelSafeGroup UIModelWorker::GetModelSafeGroup() {
   return GROUP_UI;
 }
 
+bool UIModelWorker::IsOnModelThread() {
+  return ui_thread_->BelongsToCurrentThread();
+}
+
 UIModelWorker::~UIModelWorker() {}
 
 }  // namespace syncer