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/common_audio/wav_file.cc

diff --git a/webrtc/common_audio/wav_file.cc b/webrtc/common_audio/wav_file.cc
index 27d335c..94b7a3c 100644
--- a/webrtc/common_audio/wav_file.cc
+++ b/webrtc/common_audio/wav_file.cc
@@ -99,7 +99,7 @@
 }
 
 WavWriter::WavWriter(const std::string& filename, int sample_rate,
-                     int num_channels)
+                     size_t num_channels)
     : sample_rate_(sample_rate),
       num_channels_(num_channels),
       num_samples_(0),
@@ -153,7 +153,7 @@
 
 rtc_WavWriter* rtc_WavOpen(const char* filename,
                            int sample_rate,
-                           int num_channels) {
+                           size_t num_channels) {
   return reinterpret_cast<rtc_WavWriter*>(
       new webrtc::WavWriter(filename, sample_rate, num_channels));
 }
@@ -172,7 +172,7 @@
   return reinterpret_cast<const webrtc::WavWriter*>(wf)->sample_rate();
 }
 
-int rtc_WavNumChannels(const rtc_WavWriter* wf) {
+size_t rtc_WavNumChannels(const rtc_WavWriter* wf) {
   return reinterpret_cast<const webrtc::WavWriter*>(wf)->num_channels();
 }