Convert channel counts to size_t.

IIRC, this was originally requested by ajm during review of the other size_t conversions I did over the past year, and I agreed it made sense, but wanted to do it separately since those changes were already gargantuan.

BUG=chromium:81439
TEST=none
R=henrik.lundin@webrtc.org, henrika@webrtc.org, kjellander@webrtc.org, minyue@webrtc.org, perkj@webrtc.org, solenberg@webrtc.org, stefan@webrtc.org, tina.legrand@webrtc.org

Review URL: https://codereview.webrtc.org/1316523002 .

Cr-Commit-Position: refs/heads/master@{#11229}

webrtc/modules/audio_coding/test/opus_test.cc

diff --git a/webrtc/modules/audio_coding/test/opus_test.cc b/webrtc/modules/audio_coding/test/opus_test.cc
index 466db9f..104b5e5 100644
--- a/webrtc/modules/audio_coding/test/opus_test.cc
+++ b/webrtc/modules/audio_coding/test/opus_test.cc
@@ -62,7 +62,7 @@
   return;
 #else
   uint16_t frequency_hz;
-  int audio_channels;
+  size_t audio_channels;
   int16_t test_cntr = 0;
 
   // Open both mono and stereo test files in 32 kHz.
@@ -205,7 +205,7 @@
 #endif
 }
 
-void OpusTest::Run(TestPackStereo* channel, int channels, int bitrate,
+void OpusTest::Run(TestPackStereo* channel, size_t channels, int bitrate,
                    size_t frame_length, int percent_loss) {
   AudioFrame audio_frame;
   int32_t out_freq_hz_b = out_file_.SamplingFrequency();