Enable the -Wundef warning for clang

webrtc/modules/video_processing/test/video_processing_unittest.h

 /*
  *  Copyright (c) 2011 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.
  */
 
 #ifndef WEBRTC_MODULES_VIDEO_PROCESSING_TEST_VIDEO_PROCESSING_UNITTEST_H_
 #define WEBRTC_MODULES_VIDEO_PROCESSING_TEST_VIDEO_PROCESSING_UNITTEST_H_
 
 #include <string>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/video_processing/include/video_processing.h"
 #include "webrtc/system_wrappers/include/trace.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
 
 class VideoProcessingTest : public ::testing::Test {
  protected:
   VideoProcessingTest();
   virtual void SetUp();
@@ skipping 10 matching lines @@
   const int half_width_;
   const int height_;
   const int size_y_;
   const int size_uv_;
   const size_t frame_length_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_VIDEO_PROCESSING_TEST_VIDEO_PROCESSING_UNITTEST_H_