Introduce dchecked_cast, and start using it

webrtc/modules/audio_coding/codecs/audio_format_conversion.cc

Index: webrtc/modules/audio_coding/codecs/audio_format_conversion.cc
diff --git a/webrtc/modules/audio_coding/codecs/audio_format_conversion.cc b/webrtc/modules/audio_coding/codecs/audio_format_conversion.cc
index 5d42409ce0d34b454111495f3d5d014bf77462f2..5a69ae431d1c3e4ead6e9ffd214a85efffc27455 100644
--- a/webrtc/modules/audio_coding/codecs/audio_format_conversion.cc
+++ b/webrtc/modules/audio_coding/codecs/audio_format_conversion.cc
@@ -34,7 +34,7 @@ CodecInst MakeCodecInst(int payload_type,
   strncpy(ci.plname, name, sizeof(ci.plname));
   ci.plname[sizeof(ci.plname) - 1] = '\0';
   ci.plfreq = sample_rate;
-  ci.channels = rtc::checked_cast<size_t>(num_channels);
+  ci.channels = rtc::dchecked_cast<size_t>(num_channels);
   return ci;
 }