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}

api/rtp_transceiver_interface.h

@@ -31,7 +31,8 @@ enum class RtpTransceiverDirection {
   kSendRecv,
   kSendOnly,
   kRecvOnly,
-  kInactive
+  kInactive,
+  kStopped,
 };
 
 // Structure for initializing an RtpTransceiver in a call to