[Sync] Replace uses of ObserverListThreadSafe with WeakHandles

chrome/browser/sync/test/null_transaction_observer.h

+// 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.
+
+#ifndef CHROME_BROWSER_SYNC_TEST_NULL_TRANSACTION_OBSERVER_H_
+#define CHROME_BROWSER_SYNC_TEST_NULL_TRANSACTION_OBSERVER_H_
+#pragma once
+
+#include "chrome/browser/sync/util/weak_handle.h"
+
+namespace syncable {
+
+class TransactionObserver;
+
+// Returns an initialized weak handle to a transaction observer that
+// does nothing.
+browser_sync::WeakHandle<TransactionObserver> NullTransactionObserver();
+
+}  // namespace syncable
+
+#endif  // CHROME_BROWSER_SYNC_TEST_NULL_TRANSACTION_OBSERVER_H_