Add WebRTC test for offer with duplicate a=msid line

This is illegal according to RFC 8830 section 2.

Bug: webrtc:15845
Change-Id: I84c70d369178c9ee65aed94407c1524a19988318
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5296402
Reviewed-by: Florent Castelli <[email protected]>
Commit-Queue: Harald Alvestrand <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1261159}

webrtc/RTCPeerConnection-setRemoteDescription-offer.html

@@ -4,6 +4,7 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="RTCPeerConnection-helper.js"></script>
+<script src="/webrtc/third_party/sdp/sdp.js"></script>
 <script>
   'use strict';
 
@@ -365,4 +366,19 @@
     assert_equals(pc2.getTransceivers().length, 1);
   }, 'Transceivers added by sRD(offer) should not show up until sRD resolves');
 
+  promise_test(async t => {
+    const pc1 = new RTCPeerConnection();
+    const pc2 = new RTCPeerConnection();
+    t.add_cleanup(() => pc1.close());
+    t.add_cleanup(() => pc2.close());
+    pc1.addTransceiver('video');
+    const description = await pc1.createOffer();
+    const sections = SDPUtils.splitSections(description.sdp);
+    // Compose SDP with a duplicated media section (equal MSID lines)
+    // This is not permitted according to RFC 8830 section 2.
+    const sdp = sections[0] + sections[1] + sections[1].replace('a=mid:', 'a=mid:unique');
+    const p = pc2.setRemoteDescription({type: 'offer', sdp: sdp});
+    await promise_rejects_dom(t, 'OperationError', p);
+  }, 'setRemoteDescription(section with duplicate msid) rejects');
+
 </script>