Migrate Extensions code to Task Scheduler API

extensions/browser/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 "extensions/browser/api/storage/backend_task_runner.h"
+
+#include "base/task_scheduler/lazy_task_runner.h"
+
+namespace extensions {
+
+base::LazySequencedTaskRunner g_sequenced_task_task_runner =
+    LAZY_SEQUENCED_TASK_RUNNER_INITIALIZER({base::MayBlock()});
+
+scoped_refptr<base::SequencedTaskRunner> GetBackendTaskRunner() {
+  return g_sequenced_task_task_runner.Get();
+}
+
+bool IsOnBackendSequence() {
+  return GetBackendTaskRunner()->RunsTasksInCurrentSequence();
+}
+
+}  // namespace extensions