Enable the -Wundef warning for clang

webrtc/modules/video_coding/codecs/vp8/screenshare_layers_unittest.cc

 /*
  *  Copyright (c) 2013 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 <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "vpx/vpx_encoder.h"
 #include "vpx/vp8cx.h"
 #include "webrtc/modules/video_coding/include/video_codec_interface.h"
 #include "webrtc/modules/video_coding/codecs/vp8/screenshare_layers.h"
 #include "webrtc/modules/video_coding/utility/mock/mock_frame_dropper.h"
 #include "webrtc/system_wrappers/include/clock.h"
 #include "webrtc/system_wrappers/include/metrics.h"
 #include "webrtc/system_wrappers/include/metrics_default.h"
 
 using ::testing::_;
@@ skipping 520 matching lines @@
             metrics::NumEvents("WebRTC.Video.Screenshare.Layer1.Qp", kTl1Qp));
   EXPECT_EQ(1,
             metrics::NumEvents("WebRTC.Video.Screenshare.Layer0.TargetBitrate",
                                kDefaultTl0BitrateKbps));
   EXPECT_EQ(1,
             metrics::NumEvents("WebRTC.Video.Screenshare.Layer1.TargetBitrate",
                                kDefaultTl1BitrateKbps));
 }
 
 }  // namespace webrtc