[Sync] Replace uses of ObserverListThreadSafe with WeakHandles

chrome/browser/sync/test/null_transaction_observer.cc

Index: chrome/browser/sync/test/null_transaction_observer.cc
diff --git a/chrome/browser/sync/test/null_transaction_observer.cc b/chrome/browser/sync/test/null_transaction_observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..4fe914fafea393ba244ddf473997260d1bb9cc21
--- /dev/null
+++ b/chrome/browser/sync/test/null_transaction_observer.cc
@@ -0,0 +1,15 @@
+// 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/test/null_transaction_observer.h"
+
+#include "base/memory/weak_ptr.h"
+
+namespace syncable {
+
+browser_sync::WeakHandle<TransactionObserver> NullTransactionObserver() {
+  return browser_sync::MakeWeakHandle(base::WeakPtr<TransactionObserver>());
+}
+
+}  // namespace syncable