Make DirectTransport::task_queue_ const

DirectTransport::task_queue_ previously could not be const, because of a hack for the deprecated version of DirectTransport. That is no longer the case.

BUG=webrtc:8125
TBR=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/3012483002
Cr-Original-Commit-Position: refs/heads/master@{#19583}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8a6241e3511af461b64c3efa0e08ee031c7ca0c2
diff --git a/test/direct_transport.h b/test/direct_transport.h
index eee94be..11b086d 100644
--- a/test/direct_transport.h
+++ b/test/direct_transport.h
@@ -68,9 +68,7 @@
   Call* const send_call_;
   Clock* const clock_;
 
-  // TODO(eladalon): Make |task_queue_| const.
-  // https://bugs.chromium.org/p/webrtc/issues/detail?id=8125
-  SingleThreadedTaskQueueForTesting* task_queue_;
+  SingleThreadedTaskQueueForTesting* const task_queue_;
   SingleThreadedTaskQueueForTesting::TaskId next_scheduled_task_
       GUARDED_BY(&sequence_checker_);