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

base/thread_task_runner_handle.h

Index: base/thread_task_runner_handle.h
diff --git a/base/thread_task_runner_handle.h b/base/thread_task_runner_handle.h
index 238435f978e167a251c77e7e38eed7c2390d6971..ecb6fa7766d40ec09c8a15f3992659c07b492a3a 100644
--- a/base/thread_task_runner_handle.h
+++ b/base/thread_task_runner_handle.h
@@ -16,6 +16,10 @@ class SingleThreadTaskRunner;
 // in thread-local storage.  Callers can then retrieve the TaskRunner
 // for the current thread by calling ThreadTaskRunnerHandle::Get().
 // At most one TaskRunner may be bound to each thread at a time.
+// Note that the returned SingleThreadTaskRunner guarantees thread affinity, and
+// therefore this method can't be called for example on a worker thread for a
+// SequencedWorkerPool. If you only require sequencing behavior (i.e. that
+// posted tasks run after each other), use SequencedTaskRunnerHandle instead.

[gab, 2015/10/27 16:57:58]

I suggest changing all of this new text with:

// Prefer SequenceTaskRunnerHandle to this unless thread affinity is required.

(i.e. I prefer a quick and strong suggestion -- as for talking about
SequencedWorkerPool I think it's implicit and worst case developers will find
out the same way you have..?)

[Bernhard Bauer, 2015/10/27 18:03:40]

Done.

 class BASE_EXPORT ThreadTaskRunnerHandle {
  public:
   // Gets the SingleThreadTaskRunner for the current thread.