Fix compilation issues of std::unique_ptr

webrtc/p2p/base/transportdescription.h

 /*
  *  Copyright 2012 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 129 matching lines @@
     return *this;
   }
 
   bool HasOption(const std::string& option) const {
     return (std::find(transport_options.begin(), transport_options.end(),
                       option) != transport_options.end());
   }
   void AddOption(const std::string& option) {
     transport_options.push_back(option);
   }
-  bool secure() const { return identity_fingerprint != NULL; }
+  bool secure() const { return identity_fingerprint != nullptr; }
 
   IceParameters GetIceParameters() {
     return IceParameters(ice_ufrag, ice_pwd, HasOption(ICE_RENOMINATION_STR));
   }
 
   static rtc::SSLFingerprint* CopyFingerprint(
       const rtc::SSLFingerprint* from) {
     if (!from)
       return NULL;
 
     return new rtc::SSLFingerprint(*from);
   }
 
   std::vector<std::string> transport_options;
   std::string ice_ufrag;
   std::string ice_pwd;
   IceMode ice_mode;
   ConnectionRole connection_role;
 
   std::unique_ptr<rtc::SSLFingerprint> identity_fingerprint;
 };
 
 }  // namespace cricket
 
 #endif  // WEBRTC_P2P_BASE_TRANSPORTDESCRIPTION_H_