Enable the -Wundef warning for clang

webrtc/common_video/i420_video_frame_unittest.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 <math.h>
 #include <string.h>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/bind.h"
 #include "webrtc/test/fake_texture_frame.h"
 #include "webrtc/test/frame_utils.h"
 #include "webrtc/video_frame.h"
 
 namespace webrtc {
 
 namespace {
 
 rtc::scoped_refptr<I420Buffer> CreateGradient(int width, int height) {
@@ skipping 310 matching lines @@
 
   // Center crop to 640 x 360 (16/9 aspect), then scale down by 2.
   rtc::scoped_refptr<I420Buffer> scaled_buffer(
       I420Buffer::Create(320, 180));
 
   scaled_buffer->CropAndScaleFrom(buf);
   CheckCrop(scaled_buffer, 0.0, 0.125, 1.0, 0.75);
 }
 
 }  // namespace webrtc