Convert channel counts to size_t.

webrtc/modules/audio_processing/level_estimator_impl.cc

Index: webrtc/modules/audio_processing/level_estimator_impl.cc
diff --git a/webrtc/modules/audio_processing/level_estimator_impl.cc b/webrtc/modules/audio_processing/level_estimator_impl.cc
index aa676a870f838d74246ecc72ced301ac7d870001..187873e33e00f2e20bb3745045eedb06c322e3eb 100644
--- a/webrtc/modules/audio_processing/level_estimator_impl.cc
+++ b/webrtc/modules/audio_processing/level_estimator_impl.cc
@@ -35,7 +35,7 @@ void LevelEstimatorImpl::ProcessStream(AudioBuffer* audio) {
     return;
   }
 
-  for (int i = 0; i < audio->num_channels(); i++) {
+  for (size_t i = 0; i < audio->num_channels(); i++) {
     rms_->Process(audio->channels_const()[i], audio->num_frames());
   }
 }