wpt: fix webrtc simulcast rids

which need an extra parenthesis since this should be an object
as shown in
  https://w3c.github.io/webrtc-pc/#rtcrtpencodingspatialsim-example*

Noticed in
  https://github.com/fippo/simulcast-playground/pull/7

BUG=None

Change-Id: I176a2919a3dbd3e6b065dca9dcef357a1e2adc01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2247035Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779785}

third_party/blink/web_tests/external/wpt/webrtc/simulcast/simulcast.js

@@ -98,7 +98,7 @@ async function negotiateSimulcastAndWaitForVideo(t, rids, pc1, pc2, codec) {
   t.add_cleanup(() => stream.getTracks().forEach(track => track.stop()));
   const transceiver = pc1.addTransceiver(stream.getVideoTracks()[0], {
     streams: [stream],
-    sendEncodings: rids.map(rid => {rid}),
+    sendEncodings: rids.map(rid => ({rid})),
   });
   if (codec) {
     preferCodec(transceiver, codec.mimeType, codec.sdpFmtpLine);