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
1 file changed