Enable periodic bitrate probing when application limited for audio BWE.

BUG=webrtc:7043

Review-Url: https://codereview.webrtc.org/2657583005
Cr-Commit-Position: refs/heads/master@{#16325}

webrtc/audio/audio_send_stream.cc

@@ -82,6 +82,7 @@ AudioSendStream::AudioSendStream(
       channel_proxy_->SetSendAudioLevelIndicationStatus(true, extension.id);
     } else if (extension.uri == RtpExtension::kTransportSequenceNumberUri) {
       channel_proxy_->EnableSendTransportSequenceNumber(extension.id);
+      congestion_controller->EnablePeriodicAlrProbing(true);
     } else {
       RTC_NOTREACHED() << "Registering unsupported RTP extension.";
     }