Fix UT failure by temporarily uncommenting

webrtc/audio/audio_send_stream.cc

Index: webrtc/audio/audio_send_stream.cc
diff --git a/webrtc/audio/audio_send_stream.cc b/webrtc/audio/audio_send_stream.cc
index d16ae181af97cd3778f76ebe728beab8932c4135..56f3a3d76279a93ed35391db7c1e57d69756b9bf 100644
--- a/webrtc/audio/audio_send_stream.cc
+++ b/webrtc/audio/audio_send_stream.cc
@@ -275,6 +275,7 @@ void AudioSendStream::OnPacketAdded(uint32_t ssrc, uint16_t seq_num) {
 void AudioSendStream::OnPacketFeedbackVector(
     const std::vector<PacketFeedback>& packet_feedback_vector) {
   // TODO(elad.alon): 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::Optional<float> plr;
   rtc::Optional<float> rplr;