Enable the -Wundef warning for clang

webrtc/modules/audio_coding/audio_network_adaptor/bitrate_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/audio_coding/audio_network_adaptor/bitrate_controller.h"
 
 namespace webrtc {
 namespace audio_network_adaptor {
 
 namespace {
 
 // L2(14B) + IPv4(20B) + UDP(8B) + RTP(12B) + SRTP_AUTH(10B) = 64B = 512 bits
 constexpr int kPacketOverheadBits = 512;
 
@@ skipping 113 matching lines @@
   // Next: decrease overall bitrate and frame length.
   overall_bitrate -= 100;
   current_overhead_bitrate = kPacketOverheadBits * 1000 / 60;
   CheckDecision(
       &controller, rtc::Optional<int>(overall_bitrate - last_overhead_bitrate),
       rtc::Optional<int>(60), overall_bitrate - current_overhead_bitrate);
 }
 
 }  // namespace audio_network_adaptor
 }  // namespace webrtc