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/talk/media/base/fakemediaengine.h b/talk/media/base/fakemediaengine.h
index f5b2174..149704f 100644
--- a/talk/media/base/fakemediaengine.h
+++ b/talk/media/base/fakemediaengine.h
@@ -368,7 +368,7 @@
     void OnData(const void* audio_data,
                 int bits_per_sample,
                 int sample_rate,
-                int number_of_channels,
+                size_t number_of_channels,
                 size_t number_of_frames) override {}
     void OnClose() override { renderer_ = NULL; }
     AudioRenderer* renderer() const { return renderer_; }