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
diff --git a/audio/audio_send_stream.cc b/audio/audio_send_stream.cc
index f1395ab..5bf6f2b 100644
--- a/audio/audio_send_stream.cc
+++ b/audio/audio_send_stream.cc
@@ -388,9 +388,7 @@
 
 void AudioSendStream::OnPacketFeedbackVector(
     const std::vector<PacketFeedback>& packet_feedback_vector) {
-  // TODO(eladalon): This fails in UT; fix and uncomment.
-  // See: https://bugs.chromium.org/p/webrtc/issues/detail?id=7405
-  // RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
+  RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
   rtc::Optional<float> plr;
   rtc::Optional<float> rplr;
   {