webrtc wpt: use unused extension id for mid when offering to receive …

…simulcast

since mid is mandatory to implement per
https://datatracker.ietf.org/doc/html/rfc8853#section-5.5

BUG=chromium:1422258

Change-Id: I1886a345c8fb3acd9389f4553ef77e48644dbad3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4574314
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1151702}

webrtc/simulcast/simulcast.js

@@ -86,6 +86,15 @@ function midToRid(description, localDescription, rids) {
     if (localMidExtension) {
       rtpParameters.headerExtensions.push(localMidExtension);
     }
+  } else {
+    // Find unused id in remote description to formally have a mid.
+    for (let id = 1; id < 15; id++) {
+      if (rtpParameters.headerExtensions.find(ext => ext.id === id) === undefined) {
+        rtpParameters.headerExtensions.push(
+          {id, uri: 'urn:ietf:params:rtp-hdrext:sdes:mid'});
+        break;
+      }
+    }
   }
 
   if (!rids) {