Make num_encoded_channels_ atomic

Ensures that the value read by the audio thread is well-defined.

Bug: b/176104610
Change-Id: I15d1901522be79703b3dc188fbe03c752be09a60
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212442
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33503}

media/engine/webrtc_voice_engine.cc

diff --git a/media/engine/webrtc_voice_engine.cc b/media/engine/webrtc_voice_engine.cc
index 3243418..e7c2be6 100644
--- a/media/engine/webrtc_voice_engine.cc
+++ b/media/engine/webrtc_voice_engine.cc
@@ -11,6 +11,7 @@
 #include "media/engine/webrtc_voice_engine.h"
 
 #include <algorithm>
+#include <atomic>
 #include <functional>
 #include <memory>
 #include <string>
@@ -1167,7 +1168,7 @@
   // TODO(webrtc:11717): Remove this once audio_network_adaptor in AudioOptions
   // has been removed.
   absl::optional<std::string> audio_network_adaptor_config_from_options_;
-  int num_encoded_channels_ = -1;
+  std::atomic<int> num_encoded_channels_{-1};
 };
 
 class WebRtcVoiceMediaChannel::WebRtcAudioReceiveStream {