Enable the -Wundef warning for clang

webrtc/modules/remote_bitrate_estimator/overuse_detector_unittest.cc

 /*
  *  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.
  */
 
 #include <math.h>
 
 #include <algorithm>
 #include <cmath>
 #include <cstdlib>
 #include <memory>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/base/random.h"
 #include "webrtc/base/rate_statistics.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/remote_bitrate_estimator/inter_arrival.h"
 #include "webrtc/modules/remote_bitrate_estimator/overuse_detector.h"
 #include "webrtc/modules/remote_bitrate_estimator/overuse_estimator.h"
 #include "webrtc/test/field_trial.h"
 
 namespace webrtc {
@@ skipping 742 matching lines @@
     if (overuse_state == kBwOverusing) {
       overuse_detected = true;
     }
     ++num_deltas;
     now_ms += 5;
   }
   EXPECT_TRUE(overuse_detected);
 }
 }  // namespace testing
 }  // namespace webrtc