Enable the -Wundef warning for clang

webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_nalu_unittest.cc

 /*
  *  Copyright (c) 2015 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/arraysize.h"
 #include "webrtc/modules/video_coding/codecs/h264/h264_video_toolbox_nalu.h"
 
 #if defined(WEBRTC_VIDEO_TOOLBOX_SUPPORTED)
 
 namespace webrtc {
 
 static const uint8_t NALU_TEST_DATA_0[] = {0xAA, 0xBB, 0xCC};
 static const uint8_t NALU_TEST_DATA_1[] = {0xDE, 0xAD, 0xBE, 0xEF};
@@ skipping 162 matching lines @@
   EXPECT_EQ(buffer_size, writer.BytesRemaining());
   EXPECT_FALSE(writer.WriteNalu(NALU_TEST_DATA_0, arraysize(NALU_TEST_DATA_0)));
   EXPECT_EQ(buffer_size, writer.BytesRemaining());
   EXPECT_EQ(0,
             memcmp(expected_buffer, buffer.get(), arraysize(expected_buffer)));
 }
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_VIDEO_TOOLBOX_SUPPORTED