RTCRtpTransceiverDirection: add "stopped" enum value.

Work on RTP header extension control requires the new
RTCRtpTransceiverDirection enum value "stopped", but it's
not possible to conditionally add the enum value under
a flag. The work to add "stopped" is underway in
RTCRtpTransceiver's new "stop" functionality so we're
landing the attribute addition under this intent to ship.

This change expands the serialization functions to allow
landing the "stopped" enum value in the WebRTC API, but
doesn't yet serialize the value.

Intent to prototype: https://groups.google.com/a/chromium.org/g/blink-dev/c/65YdUi02yZk
Bug: 980879

Change-Id: I32833ff16720a002b193b33e71c2fb493227002e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2093219
Commit-Queue: Markus Handell <handellm@google.com>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748233}

third_party/blink/renderer/modules/peerconnection/peer_connection_tracker.cc

@@ -144,6 +144,9 @@ String SerializeDirection(webrtc::RtpTransceiverDirection direction) {
       return "'recvonly'";
     case webrtc::RtpTransceiverDirection::kInactive:
       return "'inactive'";
+    default:
+      NOTREACHED();
+      return String();
   }
 }
 

third_party/blink/renderer/modules/peerconnection/rtc_rtp_transceiver.cc

@@ -28,6 +28,9 @@ String TransceiverDirectionToString(
       return "recvonly";
     case webrtc::RtpTransceiverDirection::kInactive:
       return "inactive";
+    default:
+      NOTREACHED();
+      return String();
   }
 }
 

third_party/blink/renderer/modules/peerconnection/rtc_rtp_transceiver.idl

@@ -7,7 +7,8 @@ enum RTCRtpTransceiverDirection {
     "sendrecv",
     "sendonly",
     "recvonly",
-    "inactive"
+    "inactive",
+    "stopped"
 };
 
 // https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiver