Add generic frame descriptor to list of supported video extensions

Update uri to include version.

Bug: webrtc:9361
Change-Id: I71164879d2a5b88c052fdab9f74202f758e85be6
Reviewed-on: https://webrtc-review.googlesource.com/101762
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24827}
diff --git a/api/rtpparameters.h b/api/rtpparameters.h
index f2e4dcb..70d7e00 100644
--- a/api/rtpparameters.h
+++ b/api/rtpparameters.h
@@ -286,6 +286,10 @@
   static const char kFrameMarkingUri[];
   static const int kFrameMarkingDefaultId;
 
+  // Experimental codec agnostic frame descriptor.
+  static const char kGenericFrameDescriptorUri[];
+  static const int kGenericFrameDescriptorDefaultId;
+
   // Header extension for transport sequence number, see url for details:
   // http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions
   static const char kTransportSequenceNumberUri[];