Remove dependency on BrowserThread::FILE from Sync code

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

Index: chrome/browser/extensions/api/storage/backend_task_runner.h
diff --git a/chrome/browser/extensions/api/storage/backend_task_runner.h b/chrome/browser/extensions/api/storage/backend_task_runner.h
new file mode 100644
index 0000000000000000000000000000000000000000..50c60e08d063ee11435562ed596b64a106e40811
--- /dev/null
+++ b/chrome/browser/extensions/api/storage/backend_task_runner.h
@@ -0,0 +1,18 @@
+// 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_