Expose codec implementation names in stats.

webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_encoder.cc

Index: webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_encoder.cc
diff --git a/webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_encoder.cc b/webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_encoder.cc
index 69e52a5b2748f255c4871c94aabfc3a155145987..acc83cd7208f93bfbb1b8340837ea2ee4d98923f 100644
--- a/webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_encoder.cc
+++ b/webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_encoder.cc
@@ -433,6 +433,10 @@ void H264VideoToolboxEncoder::DestroyCompressionSession() {
   }
 }
 
+const char* H264VideoToolboxEncoder::ImplementationName() const {
+  return "VideoToolbox";
+}
+
 }  // namespace webrtc
 
 #endif  // defined(WEBRTC_VIDEO_TOOLBOX_SUPPORTED)