RtpTransceiver: add kStopped enumeration value.

This change introduces a new kStopped enumeration value to
RtpTransceiverDirection, preparing for later CLs which
implement RTP header extension control,
https://chromestatus.com/feature/5680189201711104.

The new enumeration value is unused in the code.

Intent to prototype: https://groups.google.com/a/chromium.org/g/blink-dev/c/65YdUi02yZk

Bug: chromium:980879
Change-Id: Id8cab9891236884542689fbf1b300e64a2cb636d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170050
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30756}

sdk/objc/api/peerconnection/RTCRtpTransceiver.h

@@ -22,6 +22,7 @@ typedef NS_ENUM(NSInteger, RTCRtpTransceiverDirection) {
   RTCRtpTransceiverDirectionSendOnly,
   RTCRtpTransceiverDirectionRecvOnly,
   RTCRtpTransceiverDirectionInactive,
+  RTCRtpTransceiverDirectionStopped
 };
 
 /** Structure for initializing an RTCRtpTransceiver in a call to

sdk/objc/api/peerconnection/RTCRtpTransceiver.mm

@@ -149,6 +149,8 @@
       return webrtc::RtpTransceiverDirection::kRecvOnly;
     case RTCRtpTransceiverDirectionInactive:
       return webrtc::RtpTransceiverDirection::kInactive;
+    case RTCRtpTransceiverDirectionStopped:
+      return webrtc::RtpTransceiverDirection::kStopped;
   }
 }
 
@@ -163,6 +165,8 @@
       return RTCRtpTransceiverDirectionRecvOnly;
     case webrtc::RtpTransceiverDirection::kInactive:
       return RTCRtpTransceiverDirectionInactive;
+    case webrtc::RtpTransceiverDirection::kStopped:
+      return RTCRtpTransceiverDirectionStopped;
   }
 }