Enable the -Wundef warning for clang

webrtc/modules/video_coding/sequence_number_util_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 <set>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/video_coding/sequence_number_util.h"
 
 namespace webrtc {
 class TestSeqNumUtil : public ::testing::Test {
  protected:
   // Can't use std::numeric_limits<unsigned long>::max() since
   // MSVC doesn't support constexpr.
   static const unsigned long ulmax = ~0ul;  // NOLINT
 };
 
@@ skipping 179 matching lines @@
   for (int i = 0; i < D / 2; ++i) {
     seq_nums_asc.insert(x);
     seq_nums_desc.insert(x);
     ASSERT_EQ(x, *seq_nums_asc.begin());
     ASSERT_EQ(x, *seq_nums_desc.rbegin());
     x = Add<D>(x, 1);
   }
 }
 
 }  // namespace webrtc