Add codec name to CodecSpecificInfo and get the codec name stats from there instead.

BUG=webrtc:5687

Review-Url: https://codereview.webrtc.org/2253563002
Cr-Commit-Position: refs/heads/master@{#14012}

webrtc/modules/video_coding/include/video_codec_interface.h

@@ -90,7 +90,9 @@ union CodecSpecificInfoUnion {
 // must be fitted with a copy-constructor. This is because it is copied
 // in the copy-constructor of VCMEncodedFrame.
 struct CodecSpecificInfo {
+  CodecSpecificInfo() : codecType(kVideoCodecUnknown), codec_name(nullptr) {}
   VideoCodecType codecType;
+  const char* codec_name;
   CodecSpecificInfoUnion codecSpecific;
 };