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

base/threading/sequenced_task_runner_handle.cc

Index: base/threading/sequenced_task_runner_handle.cc
diff --git a/base/threading/sequenced_task_runner_handle.cc b/base/threading/sequenced_task_runner_handle.cc
new file mode 100644
index 0000000000000000000000000000000000000000..a03642de42061cef29a2842fee5abbbc9a17df44
--- /dev/null
+++ b/base/threading/sequenced_task_runner_handle.cc
@@ -0,0 +1,38 @@
+// 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();
+    DCHECK(sequence_token.IsValid());

[gab, 2015/11/02 18:42:40]

Actually, if the |sequence_token| is invalid: doesn't that mean that the current
task was posted one-off (not on a sequence)? If so, we should still return a
valid SequenceTaskRunner (with a new token) here, right?

[danakj, 2015/11/02 18:44:52]

Maybe I'm confused, but I thought the idea was that you'd use this only from
within a task running on a SWP, which means it would be valid. Are you saying
you can call this from any place and expect some SWP to pick it up? (which one?)

+    DCHECK(pool->IsRunningSequenceOnCurrentThread(sequence_token));
+    return pool->GetSequencedTaskRunner(sequence_token);
+  }
+
+  // Otherwise, return a SingleThreadTaskRunner for the current thread.
+  return base::ThreadTaskRunnerHandle::Get();
+}
+
+// static
+bool SequencedTaskRunnerHandle::IsSet() {
+  return (SequencedWorkerPool::GetWorkerPoolForCurrentThread() &&
+          SequencedWorkerPool::GetSequenceTokenForCurrentThread().IsValid()) ||
+         base::ThreadTaskRunnerHandle::IsSet();
+}
+
+}  // namespace base