Remove dependency on BrowserThread::FILE from Sync code

chrome/browser/extensions/api/storage/backend_task_runner.cc

+// Copyright 2017 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/extensions/api/storage/backend_task_runner.h"
+
+#include "content/public/browser/browser_thread.h"
+
+namespace extensions {
+
+// TODO(stanisc): crbug.com/731903: change to LazySequencedTaskRunner once
+// migration from BrowserThread::FILE to GetBackendTaskRunner() is complete.
+scoped_refptr<base::SequencedTaskRunner> GetBackendTaskRunner() {
+  return content::BrowserThread::GetTaskRunnerForThread(
+      content::BrowserThread::FILE);
+}
+
+}  // namespace extensions