Enable the -Wundef warning for clang

webrtc/modules/video_coding/codecs/vp8/vp8_sequence_coder.cc

 /*
  *  Copyright (c) 2012 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/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/checks.h"
 #include "webrtc/base/timeutils.h"
 #include "webrtc/common_video/include/video_image.h"
 #include "webrtc/common_video/libyuv/include/webrtc_libyuv.h"
 #include "webrtc/modules/video_coding/codecs/vp8/include/vp8.h"
 #include "webrtc/test/testsupport/fileutils.h"
 #include "webrtc/test/testsupport/metrics/video_metrics.h"
 #include "webrtc/tools/simple_command_line_parser.h"
 #include "webrtc/video_frame.h"
 
@@ skipping 219 matching lines @@
 
   parser.ProcessFlags();
   if (parser.GetFlag("help") == "true") {
     parser.PrintUsageMessage();
     exit(EXIT_SUCCESS);
   }
   parser.PrintEnteredFlags();
 
   return SequenceCoder(&parser);
 }