Diff serv build fix for iOS (#141)

Author: taste1981

sdk/objc/api/peerconnection/RTCRtpCodecParameters.mm

@@ -59,6 +59,9 @@ - (instancetype)initWithNativeParameters:
       case cricket::MEDIA_TYPE_VIDEO:
         _kind = kRTCMediaStreamTrackKindVideo;
         break;
+      case cricket::MEDIA_TYPE_SCREEN:
+        _kind = kRTCMediaStreamTrackKindVideo;
+        break;
       case cricket::MEDIA_TYPE_DATA:
         RTC_NOTREACHED();
         break;

sdk/objc/api/peerconnection/RTCRtpReceiver.mm

@@ -125,6 +125,8 @@ + (RTCRtpMediaType)mediaTypeForNativeMediaType:
       return RTCRtpMediaTypeVideo;
     case cricket::MEDIA_TYPE_DATA:
       return RTCRtpMediaTypeData;
+    case cricket::MEDIA_TYPE_SCREEN:
+      return RTCRtpMediaTypeVideo;
     case cricket::MEDIA_TYPE_UNSUPPORTED:
       return RTCRtpMediaTypeUnsupported;
   }