Prevents issue in webrtc::ValidateLayout for devices with more than 8 output channels

It seems like there are some audio devices with more than 8 channels (on Mac) trying to
render remote media streams. Given that there is no standards on how to upmix to more
than 8 channels we instead provide silence in these extreme cases instead of crashing
which was the case before.

Bug: 986415
Change-Id: I82d9422e4888cb07ccc1c531e3276f2685bff6cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1751191Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Andreasson <henrika@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686395}

third_party/blink/renderer/modules/webrtc/webrtc_audio_device_impl.cc

@@ -56,9 +56,13 @@ void WebRtcAudioDeviceImpl::RenderData(media::AudioBus* audio_bus,
         sink->OnRenderThreadChanged();
     }
 #endif
-    if (!playing_) {
+    if (!playing_ || audio_bus->channels() > 8) {
       // Force silence to AudioBus after stopping playout in case
-      // there is lingering audio data in AudioBus.
+      // there is lingering audio data in AudioBus or if the audio device has
+      // more than eight channels (which is not supported by the channel mixer
+      // in WebRTC).
+      // See http://crbug.com/986415 for details on why the extra check for
+      // number of channels is required.
       audio_bus->Zero();
       return;
     }