Piggybacking simulcast id and ALR experiment id into video content type extension.

webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc

Index: webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
diff --git a/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc b/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
index dc8bba22e036e4680c92886dd96f52188d2844ff..08b8183a54c6ca41a615c6997d8714d947b46801 100644
--- a/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
+++ b/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
@@ -706,8 +706,8 @@ int VP9EncoderImpl::GetEncodedLayerFrame(const vpx_codec_cx_pkt* pkt) {
     encoded_image_.capture_time_ms_ = input_image_->render_time_ms();
     encoded_image_.rotation_ = input_image_->rotation();
     encoded_image_.content_type_ = (codec_.mode == kScreensharing)
-                                       ? VideoContentType::SCREENSHARE
-                                       : VideoContentType::UNSPECIFIED;
+                                       ? VideoContentType::Screenshare()
+                                       : VideoContentType::Unspecified();
     encoded_image_._encodedHeight = raw_->d_h;
     encoded_image_._encodedWidth = raw_->d_w;
     encoded_image_.timing_.flags = TimingFrameFlags::kInvalid;