Enable the -Wundef warning for clang

webrtc/modules/video_coding/utility/ivf_file_writer_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 "webrtc/modules/video_coding/utility/ivf_file_writer.h"
 
 #include <memory>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/helpers.h"
 #include "webrtc/base/logging.h"
 #include "webrtc/base/thread.h"
 #include "webrtc/base/timeutils.h"
 #include "webrtc/modules/rtp_rtcp/source/byte_io.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
 
 namespace {
@@ skipping 148 matching lines @@
   ASSERT_FALSE(file_writer_->Close());
 
   rtc::File out_file = rtc::File::Open(file_name_);
   VerifyIvfHeader(&out_file, fourcc, kWidth, kHeight, kNumFramesToFit, true);
   VerifyDummyTestFrames(&out_file, kNumFramesToFit);
 
   out_file.Close();
 }
 
 }  // namespace webrtc