Allow more than 2 input channels in AudioProcessing.

webrtc/common_audio/audio_util.cc

Index: webrtc/common_audio/audio_util.cc
diff --git a/webrtc/common_audio/audio_util.cc b/webrtc/common_audio/audio_util.cc
index 2047295cb9142acb794cad2da4cc688734fd14c2..be67c48f6053990ae5c16458980936532166e8a9 100644
--- a/webrtc/common_audio/audio_util.cc
+++ b/webrtc/common_audio/audio_util.cc
@@ -39,4 +39,13 @@ void FloatS16ToFloat(const float* src, size_t size, float* dest) {
     dest[i] = FloatS16ToFloat(src[i]);
 }
 
+template <>
+void DownmixInterleavedToMono<int16_t>(const int16_t* interleaved,
+                                       int num_multichannel_frames,
+                                       int num_channels,
+                                       int16_t* deinterleaved) {
+  DownmixInterleavedToMonoImpl<int16_t, int32_t>(
+      interleaved, num_multichannel_frames, num_channels, deinterleaved);
+}
+
 }  // namespace webrtc