Enable the -Wundef warning for clang

webrtc/test/rtp_rtcp_observer.h

 /*
  *  Copyright (c) 2013 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.
  */
 #ifndef WEBRTC_TEST_RTP_RTCP_OBSERVER_H_
 #define WEBRTC_TEST_RTP_RTCP_OBSERVER_H_
 
 #include <map>
 #include <memory>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/base/criticalsection.h"
 #include "webrtc/base/event.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_header_parser.h"
 #include "webrtc/test/constants.h"
 #include "webrtc/test/direct_transport.h"
 #include "webrtc/typedefs.h"
 #include "webrtc/video_send_stream.h"
 
 namespace webrtc {
@@ skipping 103 matching lines @@
     return true;  // Will never happen, makes compiler happy.
   }
 
   RtpRtcpObserver* const observer_;
   TransportType transport_type_;
 };
 }  // namespace test
 }  // namespace webrtc
 
 #endif  // WEBRTC_TEST_RTP_RTCP_OBSERVER_H_