Enable the -Wundef warning for clang

webrtc/modules/video_coding/utility/frame_dropper_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 "webrtc/modules/video_coding/utility/frame_dropper.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/logging.h"
 
 namespace webrtc {
 
 namespace {
 
 const float kTargetBitRateKbps = 300;
 const float kIncomingFrameRate = 30;
 const size_t kFrameSizeBytes = 1250;
 
@@ skipping 128 matching lines @@
 TEST_F(FrameDropperTest, TrafficVolumeAboveAvailableBandwidth) {
   ValidateThroughputMatchesTargetBitrate(700, kIncludeKeyFrame);
   ValidateThroughputMatchesTargetBitrate(700, kDoNotIncludeKeyFrame);
   ValidateThroughputMatchesTargetBitrate(600, kIncludeKeyFrame);
   ValidateThroughputMatchesTargetBitrate(600, kDoNotIncludeKeyFrame);
   ValidateThroughputMatchesTargetBitrate(500, kIncludeKeyFrame);
   ValidateThroughputMatchesTargetBitrate(500, kDoNotIncludeKeyFrame);
 }
 
 }  // namespace webrtc