Enable the -Wundef warning for clang

webrtc/modules/video_coding/codecs/vp8/simulcast_unittest.h

 /*
  *  Copyright (c) 2014 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.
  */
 
 #ifndef WEBRTC_MODULES_VIDEO_CODING_CODECS_VP8_SIMULCAST_UNITTEST_H_
 #define WEBRTC_MODULES_VIDEO_CODING_CODECS_VP8_SIMULCAST_UNITTEST_H_
 
 #include <algorithm>
 #include <memory>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/checks.h"
 #include "webrtc/common_video/libyuv/include/webrtc_libyuv.h"
 #include "webrtc/modules/video_coding/include/mock/mock_video_codec_interface.h"
 #include "webrtc/modules/video_coding/codecs/vp8/include/vp8.h"
 #include "webrtc/modules/video_coding/codecs/vp8/temporal_layers.h"
 #include "webrtc/video_frame.h"
 
 using ::testing::_;
 using ::testing::AllOf;
 using ::testing::Field;
@@ skipping 942 matching lines @@
   MockDecodedImageCallback decoder_callback_;
   VideoCodec settings_;
   rtc::scoped_refptr<I420Buffer> input_buffer_;
   std::unique_ptr<VideoFrame> input_frame_;
 };
 
 }  // namespace testing
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_VIDEO_CODING_CODECS_VP8_SIMULCAST_UNITTEST_H_