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}
diff --git a/pc/webrtc_sdp.cc b/pc/webrtc_sdp.cc
index 29a9030..7846e5e 100644
--- a/pc/webrtc_sdp.cc
+++ b/pc/webrtc_sdp.cc
@@ -1596,7 +1596,12 @@
       InitAttrLine(kAttributeRecvOnly, &os);
       break;
     case RtpTransceiverDirection::kSendRecv:
+      InitAttrLine(kAttributeSendRecv, &os);
+      break;
+    case RtpTransceiverDirection::kStopped:
     default:
+      // kStopped shouldn't be used in signalling.
+      RTC_NOTREACHED();
       InitAttrLine(kAttributeSendRecv, &os);
       break;
   }