Reference a bug in a comment for a disabled data channel test.

webrtc/api/peerconnectionendtoend_unittest.cc

Index: webrtc/api/peerconnectionendtoend_unittest.cc
diff --git a/webrtc/api/peerconnectionendtoend_unittest.cc b/webrtc/api/peerconnectionendtoend_unittest.cc
index 2298cf482ccb57f78ec52edd7caa3d41e1eda074..80485e3f824f296312407ab5513c6530bfae38b4 100644
--- a/webrtc/api/peerconnectionendtoend_unittest.cc
+++ b/webrtc/api/peerconnectionendtoend_unittest.cc
@@ -401,6 +401,7 @@ TEST_F(PeerConnectionEndToEndTest, MessageTransferBetweenQuicDataChannels) {
 // TODO(deadbeef): This is disabled because there's currently a race condition
 // caused by the fact that a data channel signals that it's closed before it
 // really is. Re-enable this test once that's fixed.
+// See: https://bugs.chromium.org/p/webrtc/issues/detail?id=4453
 TEST_F(PeerConnectionEndToEndTest,
        DISABLED_DataChannelFromOpenWorksAfterClose) {
   MAYBE_SKIP_TEST(rtc::SSLStreamAdapter::HaveDtlsSrtp);