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/rtp_rtcp/source/rtp_receiver_impl.h

@@ -36,7 +36,7 @@ class RtpReceiverImpl : public RtpReceiver {
   int32_t RegisterReceivePayload(const char payload_name[RTP_PAYLOAD_NAME_SIZE],
                                  const int8_t payload_type,
                                  const uint32_t frequency,
-                                 const uint8_t channels,
+                                 const size_t channels,
                                  const uint32_t rate) override;
 
   int32_t DeRegisterReceivePayload(const int8_t payload_type) override;