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..197669ed88b8485ff35c0d4e14fd16cf007547fd 100644
--- a/base/thread_task_runner_handle.h
+++ b/base/thread_task_runner_handle.h
@@ -16,6 +16,7 @@ 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.
+// Prefer SequenceTaskRunnerHandle to this unless thread affinity is required.
 class BASE_EXPORT ThreadTaskRunnerHandle {
  public:
   // Gets the SingleThreadTaskRunner for the current thread.