introduce an unsupported content description type

This carries around unsupported content descriptions
(i.e. things where webrtc does not understand the media type
or protocol) in a special data type so that a rejected content or
mediasection is added to the answer SDP.

BUG=webrtc:3513

Change-Id: Ifc4168eae11e899f2504649de5e1eecb6801a9fb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/179082
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#32410}

sdk/objc/api/peerconnection/RTCRtpReceiver.h

@@ -21,6 +21,7 @@ typedef NS_ENUM(NSInteger, RTCRtpMediaType) {
   RTCRtpMediaTypeAudio,
   RTCRtpMediaTypeVideo,
   RTCRtpMediaTypeData,
+  RTCRtpMediaTypeUnsupported,
 };
 
 @class RTC_OBJC_TYPE(RTCRtpReceiver);