message: Stop inheriting from has_slots in DtlsTransport.
  Cr-Commit-Position: refs/heads/master@{#33412}
diff: #include "p2p/base/dtls_transport_internal.h"
  #include "pc/ice_transport.h"
  #include "rtc_base/synchronization/mutex.h"
  -#include "rtc_base/third_party/sigslot/sigslot.h"
  #include "rtc_base/thread.h"
  #include "rtc_base/thread_annotations.h"
  @@ -30,8 +29,7 @@ class IceTransportWithPointer;
  // This implementation wraps a cricket::DtlsTransport, and takes
  // ownership of it.
  -class DtlsTransport : public DtlsTransportInterface,
  - public sigslot::has_slots<> {
  +class DtlsTransport : public DtlsTransportInterface {
  public:
  // This object must be constructed and updated on a consistent thread,
  // the same thread as the one the cricket::DtlsTransportInternal object
  