Enable the -Wundef warning for clang

webrtc/media/engine/nullwebrtcvideoengine_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/media/engine/nullwebrtcvideoengine.h"
 #include "webrtc/media/engine/webrtcvoiceengine.h"
 #include "webrtc/modules/audio_coding/codecs/mock/mock_audio_decoder_factory.h"
 
 namespace cricket {
 
 class WebRtcMediaEngineNullVideo
     : public CompositeMediaEngine<WebRtcVoiceEngine, NullWebRtcVideoEngine> {
  public:
@@ skipping 13 matching lines @@
 // Simple test to check if NullWebRtcVideoEngine implements the methods
 // required by CompositeMediaEngine.
 TEST(NullWebRtcVideoEngineTest, CheckInterface) {
   WebRtcMediaEngineNullVideo engine(
       nullptr, webrtc::MockAudioDecoderFactory::CreateUnusedFactory(), nullptr,
       nullptr);
   EXPECT_TRUE(engine.Init());
 }
 
 }  // namespace cricket