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

webrtc/modules/video_coding/codecs/h264/h264_encoder_impl.cc

Index: webrtc/modules/video_coding/codecs/h264/h264_encoder_impl.cc
diff --git a/webrtc/modules/video_coding/codecs/h264/h264_encoder_impl.cc b/webrtc/modules/video_coding/codecs/h264/h264_encoder_impl.cc
index 84bfafb8b08b10cbbd719a57dedc7e8839bea169..315d347c69316a99ec4b5a7a2a77bd1a16a1d0cf 100644
--- a/webrtc/modules/video_coding/codecs/h264/h264_encoder_impl.cc
+++ b/webrtc/modules/video_coding/codecs/h264/h264_encoder_impl.cc
@@ -367,6 +367,9 @@
   encoded_image_.ntp_time_ms_ = input_frame.ntp_time_ms();
   encoded_image_.capture_time_ms_ = input_frame.render_time_ms();
   encoded_image_.rotation_ = input_frame.rotation();
+  encoded_image_.content_type_ = (mode_ == kScreensharing)
+                                     ? VideoContentType::SCREENSHARE
+                                     : VideoContentType::UNSPECIFIED;
   encoded_image_._frameType = ConvertToVideoFrameType(info.eFrameType);
 
   // Split encoded image up into fragments. This also updates |encoded_image_|.