Uncomment thread-check in AudioSendStream::OnPacketFeedbackVector()

The thread-check should be pass unit-tests, following https://codereview.webrtc.org/2998923002/.

BUG=webrtc:7405
TBR=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/3001313002
Cr-Original-Commit-Position: refs/heads/master@{#19498}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3651fdd1372b027a81b5652abe93b5c6ea3de442
1 file changed