Enable the -Wundef warning for clang

webrtc/modules/remote_bitrate_estimator/include/mock/mock_remote_bitrate_observer.h

 /*
  *  Copyright (c) 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.
  */
 
 #ifndef WEBRTC_MODULES_REMOTE_BITRATE_ESTIMATOR_INCLUDE_MOCK_MOCK_REMOTE_BITRATE_OBSERVER_H_
 #define WEBRTC_MODULES_REMOTE_BITRATE_ESTIMATOR_INCLUDE_MOCK_MOCK_REMOTE_BITRATE_OBSERVER_H_
 
 #include <vector>
 
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/test/gmock.h"
 #include "webrtc/modules/remote_bitrate_estimator/include/remote_bitrate_estimator.h"
 
 namespace webrtc {
 
 class MockRemoteBitrateObserver : public RemoteBitrateObserver {
  public:
   MOCK_METHOD2(OnReceiveBitrateChanged,
       void(const std::vector<unsigned int>& ssrcs, unsigned int bitrate));
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_REMOTE_BITRATE_ESTIMATOR_INCLUDE_MOCK_MOCK_REMOTE_BITRATE_OBSERVER_H_