Enable the -Wundef warning for clang

webrtc/modules/video_coding/rtp_frame_reference_finder_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 <cstring>
 #include <limits>
 #include <map>
 #include <set>
 #include <utility>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/random.h"
 #include "webrtc/base/refcount.h"
 #include "webrtc/modules/video_coding/frame_object.h"
 #include "webrtc/modules/video_coding/packet_buffer.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace webrtc {
 namespace video_coding {
 
 class FakePacketBuffer : public PacketBuffer {
@@ skipping 1176 matching lines @@
   CheckReferencesVp9(pid + 5, 1, pid + 4);
   CheckReferencesVp9(pid + 6, 0, pid + 4);
   CheckReferencesVp9(pid + 6, 1, pid + 5);
   CheckReferencesVp9(pid + 7, 1, pid + 6);
   CheckReferencesVp9(pid + 8, 0, pid + 6);
   CheckReferencesVp9(pid + 8, 1, pid + 7);
 }
 
 }  // namespace video_coding
 }  // namespace webrtc