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}
diff --git a/webrtc/audio/audio_sink.h b/webrtc/audio/audio_sink.h
index d022b32..999644f 100644
--- a/webrtc/audio/audio_sink.h
+++ b/webrtc/audio/audio_sink.h
@@ -30,7 +30,7 @@
     Data(int16_t* data,
          size_t samples_per_channel,
          int sample_rate,
-         int channels,
+         size_t channels,
          uint32_t timestamp)
         : data(data),
           samples_per_channel(samples_per_channel),
@@ -41,7 +41,7 @@
     int16_t* data;               // The actual 16bit audio data.
     size_t samples_per_channel;  // Number of frames in the buffer.
     int sample_rate;             // Sample rate in Hz.
-    int channels;                // Number of channels in the audio data.
+    size_t channels;             // Number of channels in the audio data.
     uint32_t timestamp;          // The RTP timestamp of the first sample.
   };