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/modules/audio_coding/test/EncodeDecodeTest.h b/webrtc/modules/audio_coding/test/EncodeDecodeTest.h
index 3881062..f9a9a5b 100644
--- a/webrtc/modules/audio_coding/test/EncodeDecodeTest.h
+++ b/webrtc/modules/audio_coding/test/EncodeDecodeTest.h
@@ -48,7 +48,7 @@
  public:
   Sender();
   void Setup(AudioCodingModule *acm, RTPStream *rtpStream,
-             std::string in_file_name, int sample_rate, int channels);
+             std::string in_file_name, int sample_rate, size_t channels);
   void Teardown();
   void Run();
   bool Add10MsData();
@@ -71,7 +71,7 @@
   Receiver();
   virtual ~Receiver() {};
   void Setup(AudioCodingModule *acm, RTPStream *rtpStream,
-             std::string out_file_name, int channels);
+             std::string out_file_name, size_t channels);
   void Teardown();
   void Run();
   virtual bool IncomingPacket();