Enable the -Wundef warning for clang

webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc

 /*
  *  Copyright (c) 2015 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.
  */
 
 #include <algorithm>
 
 #include "webrtc/modules/remote_bitrate_estimator/test/estimators/remb.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/common.h"
 #include "webrtc/modules/bitrate_controller/include/bitrate_controller.h"
 #include "webrtc/modules/remote_bitrate_estimator/remote_bitrate_estimator_abs_send_time.h"
 #include "webrtc/modules/remote_bitrate_estimator/test/bwe_test_logging.h"
 #include "webrtc/modules/rtp_rtcp/include/receive_statistics.h"
 
 namespace webrtc {
 namespace testing {
 namespace bwe {
 
@@ skipping 128 matching lines @@
     }
     latest_estimate_bps_ = bps;
   }
   *estimate_bps = latest_estimate_bps_;
   return true;
 }
 
 }  // namespace bwe
 }  // namespace testing
 }  // namespace webrtc