Remove dependency on BrowserThread::FILE from Sync code

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

+// 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.
+
+#ifndef CHROME_BROWSER_EXTENSIONS_API_STORAGE_BACKEND_TASK_RUNNER_H_
+#define CHROME_BROWSER_EXTENSIONS_API_STORAGE_BACKEND_TASK_RUNNER_H_
+
+#include "base/memory/ref_counted.h"
+#include "base/sequenced_task_runner.h"
+
+namespace extensions {
+
+// Gets the singleton task runner for running SyncStorageBackend on.
+scoped_refptr<base::SequencedTaskRunner> GetBackendTaskRunner();
+
+}  // namespace extensions
+
+#endif  // CHROME_BROWSER_EXTENSIONS_API_STORAGE_BACKEND_TASK_RUNNER_H_