Enable the -Wundef warning for clang

webrtc/modules/audio_coding/audio_network_adaptor/frame_length_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 <utility>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/audio_network_adaptor/frame_length_controller.h"
 
 namespace webrtc {
 
 namespace {
 
 constexpr float kFlIncreasingPacketLossFraction = 0.04f;
 constexpr float kFlDecreasingPacketLossFraction = 0.05f;
 constexpr int kFl20msTo60msBandwidthBps = 22000;
 constexpr int kFl60msTo20msBandwidthBps = 88000;
@@ skipping 267 matching lines @@
   CheckDecision(controller.get(),
                 rtc::Optional<int>(kFl120msTo60msBandwidthBps),
                 rtc::Optional<float>(kFlIncreasingPacketLossFraction),
                 rtc::Optional<bool>(), 60);
   CheckDecision(controller.get(), rtc::Optional<int>(kMediumPacketLossFraction),
                 rtc::Optional<float>(kFlDecreasingPacketLossFraction),
                 rtc::Optional<bool>(), 20);
 }
 
 }  // namespace webrtc