Enable the -Wundef warning for clang

webrtc/modules/video_coding/codecs/vp9/vp9_screenshare_layers_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 <limits>
 #include <memory>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "vpx/vp8cx.h"
 #include "webrtc/base/logging.h"
 #include "webrtc/modules/video_coding/codecs/vp9/screenshare_layers.h"
 #include "webrtc/modules/video_coding/codecs/vp9/vp9_impl.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace webrtc {
 
 typedef VP9EncoderImpl::SuperFrameRefSettings Settings;
 
@@ skipping 290 matching lines @@
   EqualToExpected();
   AddKilobitsToLayer(200, 0);
 
   // Wrap
   expected_.start_layer = 1;
   AdvanceTime(1);
   EqualToExpected();
 }
 
 }  // namespace webrtc