Bug 1539809 - Part 0: Add a wpt that triggers this bug. r=jib

Differential Revision: https://phabricator.services.mozilla.com/D25443

--HG--
extra : moz-landing-system : lando

testing/web-platform/tests/webrtc/RTCPeerConnection-setRemoteDescription-rollback.html

@@ -132,4 +132,31 @@
     await pc1.setLocalDescription(offer1);
   }, `local offer created before setRemoteDescription(remote offer) then rollback should still be usable`);
 
+  promise_test(async t => {
+    const pc1 = new RTCPeerConnection();
+    const pc2 = new RTCPeerConnection();
+    t.add_cleanup(() => pc1.close());
+    t.add_cleanup(() => pc2.close());
+
+    const stream = await getNoiseStream({ video: true });
+    t.add_cleanup(() => stream.getTracks().forEach(track => track.stop()));
+    pc1.addTrack(stream.getTracks()[0], stream);
+
+    // We don't use this right away. pc1 has provisionally decided that the
+    // (only) transceiver is bound to level 0.
+    const offer1 = await pc1.createOffer();
+
+    // Create offer from pc2, apply and rollback on pc1
+    const offer2 = await pc2.createOffer({offerToReceiveAudio: true, offerToReceiveVideo: true});
+    // pc1 now should change its mind about what level its video transceiver is
+    // bound to. It was 0, now it is 1.
+    await pc1.setRemoteDescription(offer2);
+
+    // Rolling back should put things back the way they were.
+    await pc1.setRemoteDescription({type: "rollback"});
+
+    // Then try applying pc1's old offer
+    await pc1.setLocalDescription(offer1);
+  }, "local offer created before setRemoteDescription(remote offer) with different transceiver level assignments then rollback should still be usable");
+
 </script>