[NOT FOR REVIEW] Convert channel counts to size_t.

webrtc/voice_engine/output_mixer.cc

Index: webrtc/voice_engine/output_mixer.cc
diff --git a/webrtc/voice_engine/output_mixer.cc b/webrtc/voice_engine/output_mixer.cc
index d0757a56136c90152c1b04560da679bdfe00a014..edb3fdb6f81ee30d6720191117e01591890999b6 100644
--- a/webrtc/voice_engine/output_mixer.cc
+++ b/webrtc/voice_engine/output_mixer.cc
@@ -10,6 +10,7 @@
 
 #include "webrtc/voice_engine/output_mixer.h"
 
+#include "webrtc/base/format_macros.h"
 #include "webrtc/modules/audio_processing/include/audio_processing.h"
 #include "webrtc/modules/utility/interface/audio_frame_operations.h"
 #include "webrtc/system_wrappers/interface/critical_section_wrapper.h"
@@ -462,11 +463,12 @@ int OutputMixer::StopRecordingPlayout()
 }
 
 int OutputMixer::GetMixedAudio(int sample_rate_hz,
-                               int num_channels,
+                               size_t num_channels,
                                AudioFrame* frame) {
-  WEBRTC_TRACE(kTraceStream, kTraceVoice, VoEId(_instanceId,-1),
-               "OutputMixer::GetMixedAudio(sample_rate_hz=%d, num_channels=%d)",
-               sample_rate_hz, num_channels);
+  WEBRTC_TRACE(
+      kTraceStream, kTraceVoice, VoEId(_instanceId,-1),
+      "OutputMixer::GetMixedAudio(sample_rate_hz=%d, num_channels=%" PRIuS ")",
+      sample_rate_hz, num_channels);
 
   // --- Record playout if enabled
   {