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

webrtc/config.h

Index: webrtc/config.h
diff --git a/webrtc/config.h b/webrtc/config.h
index f8c9e8b7971724835a4b62b47b038ed40a61a903..f0039b3a7263c1f14e0851cb9a97a77fc686f076 100644
--- a/webrtc/config.h
+++ b/webrtc/config.h
@@ -87,6 +87,10 @@
   // http://www.etsi.org/deliver/etsi_ts/126100_126199/126114/12.07.00_60/ts_126114v120700p.pdf
   static const char* kVideoRotationUri;
   static const int kVideoRotationDefaultId;
+
+  // Header extension for video content type. E.g. default or screenshare.
+  static const char* kVideoContentTypeUri;
+  static const int kVideoContentTypeDefaultId;
 
   // Header extension for transport sequence number, see url for details:
   // http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions