Test handling of rejected m-lines without transport description

adds a unit test for
  https://webrtc-review.googlesource.com/c/src/+/340322
which is a single m-line variant of the original
fiddle that does not require renegotiation

BUG=chromium:326493639

Change-Id: Icc5ebb1dda6502b00828a77e13b9f5fc865d34c4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/340500
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41818}

pc/sdp_offer_answer_unittest.cc

diff --git a/pc/sdp_offer_answer_unittest.cc b/pc/sdp_offer_answer_unittest.cc
index f4c35bf..2e77f88 100644
--- a/pc/sdp_offer_answer_unittest.cc
+++ b/pc/sdp_offer_answer_unittest.cc
@@ -1215,4 +1215,31 @@
             cricket::kMsidSignalingNotUsed);
 }
 
+TEST_F(SdpOfferAnswerTest, OfferWithRejectedMlineWithoutFingerprintIsAccepted) {
+  auto pc = CreatePeerConnection();
+  // A rejected m-line without fingerprint.
+  // The answer does not require one.
+  std::string sdp =
+      "v=0\r\n"
+      "o=- 0 3 IN IP4 127.0.0.1\r\n"
+      "s=-\r\n"
+      "t=0 0\r\n"
+      "a=setup:actpass\r\n"
+      "a=ice-ufrag:ETEn\r\n"
+      "a=ice-pwd:OtSK0WpNtpUjkY4+86js7Z/l\r\n"
+      "m=audio 0 RTP/SAVPF 111\r\n"
+      "c=IN IP4 0.0.0.0\r\n"
+      "a=sendrecv\r\n"
+      "a=rtpmap:111 opus/48000/2\r\n"
+      "a=rtcp-mux\r\n";
+  auto desc = CreateSessionDescription(SdpType::kOffer, sdp);
+  ASSERT_NE(desc, nullptr);
+  RTCError error;
+  pc->SetRemoteDescription(std::move(desc), &error);
+  EXPECT_TRUE(error.ok());
+
+  auto answer = pc->CreateAnswer();
+  EXPECT_TRUE(pc->SetLocalDescription(std::move(answer)));
+}
+
 }  // namespace webrtc