Allow DTLSTransport state change from "closed" to "closed"

In certain cases involving stopped transceivers, a sender or
receiver can be created with an already closed DTLSTransport,
which will cause the state to be set to "closed" even when it
is already closed.
Allow this case.

Bug: webrtc:11840
Change-Id: I21aad316f5cf74a6b4bf18ac87fcf460afc74531
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424260Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809770}

third_party/blink/renderer/modules/peerconnection/rtc_dtls_transport.cc

@@ -94,7 +94,8 @@ webrtc::DtlsTransportInterface* RTCDtlsTransport::native_transport() {
 }
 
 void RTCDtlsTransport::ChangeState(webrtc::DtlsTransportInformation info) {
-  DCHECK(current_state_.state() != webrtc::DtlsTransportState::kClosed);
+  DCHECK(info.state() == webrtc::DtlsTransportState::kClosed ||
+         current_state_.state() != webrtc::DtlsTransportState::kClosed);
   current_state_ = info;
 }