Add SequencedTaskRunnerHandle to get a SequencedTaskRunner for the current thread / sequence.

base/threading/sequenced_task_runner_handle.cc

+// Copyright 2015 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 "base/threading/sequenced_task_runner_handle.h"
+
+#include "base/sequenced_task_runner.h"
+#include "base/thread_task_runner_handle.h"
+#include "base/threading/sequenced_worker_pool.h"
+
+namespace base {
+
+// static
+scoped_refptr<SequencedTaskRunner> SequencedTaskRunnerHandle::Get() {
+  // If we are on a worker thread for a SequencedBlockingPool that is running a
+  // sequenced task, return a SequencedTaskRunner for it.
+  scoped_refptr<base::SequencedWorkerPool> pool =
+      SequencedWorkerPool::GetWorkerPoolForCurrentThread();
+  if (pool) {
+    SequencedWorkerPool::SequenceToken sequence_token =
+        SequencedWorkerPool::GetSequenceTokenForCurrentThread();
+    if (!sequence_token.IsValid())
+      return nullptr;
+
+    DCHECK(pool->IsRunningSequenceOnCurrentThread(sequence_token));
+    return pool->GetSequencedTaskRunner(sequence_token);
+  }
+
+  // Otherwise, return a SingleThreadTaskRunner for the current thread.
+  if (base::ThreadTaskRunnerHandle::IsSet())
+    return base::ThreadTaskRunnerHandle::Get();
+
+  return nullptr;
+}
+
+}  // namespace base