Fix an implicit narrowing conversion found by MSVC

webrtc/api/audio_codecs/L16/audio_decoder_L16.cc

 /*
  *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/api/audio_codecs/L16/audio_decoder_L16.h"
 
 #include "webrtc/common_types.h"
 #include "webrtc/modules/audio_coding/codecs/pcm16b/audio_decoder_pcm16b.h"
 #include "webrtc/modules/audio_coding/codecs/pcm16b/pcm16b_common.h"
 #include "webrtc/rtc_base/ptr_util.h"
+#include "webrtc/rtc_base/safe_conversions.h"
 
 namespace webrtc {
 
 rtc::Optional<AudioDecoderL16::Config> AudioDecoderL16::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>();
 }
 
 void AudioDecoderL16::AppendSupportedDecoders(
     std::vector<AudioCodecSpec>* specs) {
   Pcm16BAppendSupportedCodecSpecs(specs);
 }
 
 std::unique_ptr<AudioDecoder> AudioDecoderL16::MakeAudioDecoder(
     const Config& config) {
   return config.IsOk() ? rtc::MakeUnique<AudioDecoderPcm16B>(
                              config.sample_rate_hz, config.num_channels)
                        : nullptr;
 }
 
 }  // namespace webrtc