Fix an implicit narrowing conversion found by MSVC

webrtc/api/audio_codecs/L16/audio_encoder_L16.cc

Index: webrtc/api/audio_codecs/L16/audio_encoder_L16.cc
diff --git a/webrtc/api/audio_codecs/L16/audio_encoder_L16.cc b/webrtc/api/audio_codecs/L16/audio_encoder_L16.cc
index bd243897a1ad235645ab3a2cf8033353080da002..fe19e9717c1cc3aa78e8954cd08b6666895a051f 100644
--- a/webrtc/api/audio_codecs/L16/audio_encoder_L16.cc
+++ b/webrtc/api/audio_codecs/L16/audio_encoder_L16.cc
@@ -21,7 +21,7 @@ rtc::Optional<AudioEncoderL16::Config> AudioEncoderL16::SdpToConfig(
     const SdpAudioFormat& format) {
   Config config;
   config.sample_rate_hz = format.clockrate_hz;
-  config.num_channels = format.num_channels;
+  config.num_channels = rtc::checked_cast<int>(format.num_channels);
   return STR_CASE_CMP(format.name.c_str(), "L16") == 0 && config.IsOk()
              ? rtc::Optional<Config>(config)
              : rtc::Optional<Config>();