Enable the -Wundef warning for clang

webrtc/modules/congestion_controller/probe_controller_unittest.cc

 /*
  *  Copyright (c) 2016 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 <memory>
 
-#include "testing/gmock/include/gmock/gmock.h"
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gmock.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/logging.h"
 #include "webrtc/modules/congestion_controller/probe_controller.h"
 #include "webrtc/modules/pacing/mock/mock_paced_sender.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 using testing::_;
 using testing::AtLeast;
 using testing::NiceMock;
 
 namespace webrtc {
@@ skipping 52 matching lines @@
                                  kMaxBitrateBps);
 
   // Advance far enough to cause a time out in waiting for probing result.
   clock_.AdvanceTimeMilliseconds(kExponentialProbingTimeoutMs);
   EXPECT_CALL(pacer_, CreateProbeCluster(2 * 1800, _)).Times(0);
   probe_controller_->SetEstimatedBitrate(1800);
 }
 
 }  // namespace test
 }  // namespace webrtc