Introduce (Un)SubscribeDtlsTransportState methods.

WebRTC is going to remove cricket::DtlsTransportState and switch to
webrtc::DtlsTransportState. This CL introduces a set of new methods to
allow to subscribe to DTLS transport state updates while WebRTC
internally migrates away from cricket::DtlsTransportState.

No-Try: True
Bug: webrtc:12762
Change-Id: I79cf48e2f7122cc76fde6e4fd1541382fd096d53
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/218607
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34075}

p2p/base/dtls_transport_internal.h

diff --git a/p2p/base/dtls_transport_internal.h b/p2p/base/dtls_transport_internal.h
index ff71196..d0af5e8 100644
--- a/p2p/base/dtls_transport_internal.h
+++ b/p2p/base/dtls_transport_internal.h
@@ -132,9 +132,27 @@
     dtls_state_callback_list_.RemoveReceivers(id);
   }
 
+  // F: void(DtlsTransportInternal*, const webrtc::DtlsTransportState)
+  template <typename F>
+  void SubscribeDtlsTransportState(F&& callback) {
+    dtls_transport_state_callback_list_.AddReceiver(std::forward<F>(callback));
+  }
+
+  template <typename F>
+  void SubscribeDtlsTransportState(const void* id, F&& callback) {
+    dtls_transport_state_callback_list_.AddReceiver(id,
+                                                    std::forward<F>(callback));
+  }
+  // Unsubscribe the subscription with given id.
+  void UnsubscribeDtlsTransportState(const void* id) {
+    dtls_transport_state_callback_list_.RemoveReceivers(id);
+  }
+
   void SendDtlsState(DtlsTransportInternal* transport,
                      DtlsTransportState state) {
     dtls_state_callback_list_.Send(transport, state);
+    dtls_transport_state_callback_list_.Send(transport,
+                                             ConvertDtlsTransportState(state));
   }
 
   // Emitted whenever the Dtls handshake failed on some transport channel.
@@ -155,8 +173,12 @@
   RTC_DISALLOW_COPY_AND_ASSIGN(DtlsTransportInternal);
   webrtc::CallbackList<const rtc::SSLHandshakeError>
       dtls_handshake_error_callback_list_;
+  // TODO(bugs.webrtc.org/12762): Remove in favor of
+  // dtls_transport_state_callback_list_.
   webrtc::CallbackList<DtlsTransportInternal*, const DtlsTransportState>
       dtls_state_callback_list_;
+  webrtc::CallbackList<DtlsTransportInternal*, const webrtc::DtlsTransportState>
+      dtls_transport_state_callback_list_;
 };
 
 }  // namespace cricket