Control combined_audio_video_bwe with config bool.

Permits setting RTP extensions for AudioReceiveStream without enabling
combined A/V BWE. This prevents spamming the log with "Failed to find
extension id:".

BUG=webrtc:4870
R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1256803004

Cr-Commit-Position: refs/heads/master@{#9633}
diff --git a/webrtc/video/bitrate_estimator_tests.cc b/webrtc/video/bitrate_estimator_tests.cc
index 872fe44..cb6ef0e 100644
--- a/webrtc/video/bitrate_estimator_tests.cc
+++ b/webrtc/video/bitrate_estimator_tests.cc
@@ -210,6 +210,7 @@
         receive_config.voe_channel_id = 0;
         receive_config.rtp.extensions.push_back(
             RtpExtension(RtpExtension::kAbsSendTime, kASTExtensionId));
+        receive_config.combined_audio_video_bwe = true;
         audio_receive_stream_ = test_->receiver_call_->CreateAudioReceiveStream(
             receive_config);
       } else {