Add WPT test case for rolling back data channel

Bug: chromium:1032987
Change-Id: I450c68280a8ab8b0b2bbaa5fa31e0245dbfba780
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066625
Commit-Queue: Eldar Rello <elrello@microsoft.com>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743154}

third_party/blink/web_tests/external/wpt/webrtc/RTCPeerConnection-setRemoteDescription-rollback.html

@@ -551,4 +551,19 @@
     pc1.removeTrack(transceiver.sender);
   }, 'removeTrack() with a sender being rolled back does not crash or throw');
 
+  promise_test(async t => {
+    const pc1 = new RTCPeerConnection();
+    t.add_cleanup(() => pc1.close());
+    const pc2 = new RTCPeerConnection();
+    t.add_cleanup(() => pc2.close());
+    pc1.addTransceiver('video');
+    const channel = pc2.createDataChannel('dummy');
+    await pc2.setLocalDescription(await pc2.createOffer());
+    await pc2.setRemoteDescription(await pc1.createOffer());
+    assert_equals(pc2.signalingState, 'have-remote-offer');
+    await pc2.setLocalDescription(await pc2.createAnswer());
+    await pc2.setLocalDescription(await pc2.createOffer());
+    assert_equals(channel.readyState, 'connecting');
+  }, 'Implicit rollback with only a datachannel works');
+
 </script>

third_party/blink/web_tests/virtual/webrtc-wpt-plan-b/external/wpt/webrtc/RTCPeerConnection-setRemoteDescription-rollback-expected.txt

@@ -18,5 +18,6 @@ FAIL two transceivers with same mids promise_test: Unhandled rejection with valu
 FAIL onremovetrack fires during remote rollback promise_test: Unhandled rejection with value: object "OperationError: Failed to execute 'setRemoteDescription' on 'RTCPeerConnection': Rollback not supported in Plan B"
 FAIL rollback of a remote offer with stream changes promise_test: Unhandled rejection with value: object "TypeError: Cannot read property 'sender' of undefined"
 FAIL removeTrack() with a sender being rolled back does not crash or throw promise_test: Unhandled rejection with value: object "InvalidStateError: Failed to execute 'addTransceiver' on 'RTCPeerConnection': This operation is only supported in 'unified-plan'."
+FAIL Implicit rollback with only a datachannel works promise_test: Unhandled rejection with value: object "InvalidStateError: Failed to execute 'addTransceiver' on 'RTCPeerConnection': This operation is only supported in 'unified-plan'."
 Harness: the test ran to completion.