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_unittest.cc b/pc/webrtc_sdp_unittest.cc
index 476955d..f79fbb8 100644
--- a/pc/webrtc_sdp_unittest.cc
+++ b/pc/webrtc_sdp_unittest.cc
@@ -979,7 +979,11 @@
       new_direction = "a=recvonly";
       break;
     case RtpTransceiverDirection::kSendRecv:
+      new_direction = "a=sendrecv";
+      break;
+    case RtpTransceiverDirection::kStopped:
     default:
+      RTC_NOTREACHED();
       new_direction = "a=sendrecv";
       break;
   }