Enable the -Wundef warning for clang

webrtc/modules/audio_mixer/test/audio_mixer_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 <string.h>
 
 #include <memory>
 #include <utility>
 
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/test/gmock.h"
 #include "webrtc/base/bind.h"
 #include "webrtc/base/thread.h"
 #include "webrtc/modules/audio_mixer/audio_mixer_defines.h"
 #include "webrtc/modules/audio_mixer/audio_mixer.h"
 
 using testing::_;
 using testing::Exactly;
 using testing::Invoke;
 using testing::Return;
 
@@ skipping 472 matching lines @@
       kAudioSources, MixerAudioSource::AudioFrameInfo::kNormal);
   frame_info[0] = MixerAudioSource::AudioFrameInfo::kMuted;
   std::fill(frames[0].data_, frames[0].data_ + kDefaultSampleRateHz / 100,
             std::numeric_limits<int16_t>::max());
   std::vector<bool> expected_status(kAudioSources, true);
   expected_status[0] = false;
 
   MixAndCompare(frames, frame_info, expected_status);
 }
 }  // namespace webrtc