Bump to WebRTC M120 release

Some API deprecation -- ExperimentalAgc and ExperimentalNs are gone.
We're continuing to carry iSAC even though it's gone upstream, but maybe
we'll want to drop that soon.

webrtc/modules/audio_processing/rms_level.cc

@@ -101,8 +101,18 @@ void RmsLevel::AnalyzeMuted(size_t length) {
 }
 
 int RmsLevel::Average() {
-  int rms = (sample_count_ == 0) ? RmsLevel::kMinLevelDb
-                                 : ComputeRms(sum_square_ / sample_count_);
+  const bool have_samples = (sample_count_ != 0);
+  int rms = have_samples ? ComputeRms(sum_square_ / sample_count_)
+                         : RmsLevel::kMinLevelDb;
+
+  // To ensure that kMinLevelDb represents digital silence (muted audio
+  // sources) we'll check here if the sum_square is actually 0. If it's not
+  // we'll bump up the return value to `kInaudibleButNotMuted`.
+  // https://datatracker.ietf.org/doc/html/rfc6464
+  if (have_samples && rms == RmsLevel::kMinLevelDb && sum_square_ != 0.0f) {
+    rms = kInaudibleButNotMuted;
+  }
+
   Reset();
   return rms;
 }