Reland of Add content type information to encoded images and corresponding rtp extension header

webrtc/modules/video_coding/generic_decoder.h

Index: webrtc/modules/video_coding/generic_decoder.h
diff --git a/webrtc/modules/video_coding/generic_decoder.h b/webrtc/modules/video_coding/generic_decoder.h
index 891ec893ff2cd98e8337152ecbc64d355a04c2ff..71b8d81c7438f2c97d63db131fcd42c86126a301 100644
--- a/webrtc/modules/video_coding/generic_decoder.h
+++ b/webrtc/modules/video_coding/generic_decoder.h
@@ -30,6 +30,7 @@
   int64_t decodeStartTimeMs;
   void* userData;
   VideoRotation rotation;
+  VideoContentType content_type;
 };
 
 class VCMDecodedFrameCallback : public DecodedImageCallback {
@@ -109,6 +110,7 @@
   VideoCodecType _codecType;
   bool _isExternal;
   bool _keyFrameDecoded;
+  VideoContentType _last_keyframe_content_type;
 };
 
 }  // namespace webrtc