Enable the -Wundef warning for clang

webrtc/media/engine/webrtcmediaengine_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/modules/audio_coding/codecs/builtin_audio_decoder_factory.h"
 #include "webrtc/media/engine/webrtcmediaengine.h"
 
 using webrtc::RtpExtension;
 
 namespace cricket {
 namespace {
 
 std::vector<RtpExtension> MakeUniqueExtensions() {
@@ skipping 165 matching lines @@
   EXPECT_TRUE(engine);
 }
 
 TEST(WebRtcMediaEngineFactoryTest, CreateWithBuiltinDecoders) {
   std::unique_ptr<MediaEngineInterface> engine(WebRtcMediaEngineFactory::Create(
       nullptr, webrtc::CreateBuiltinAudioDecoderFactory(), nullptr, nullptr));
   EXPECT_TRUE(engine);
 }
 
 }  // namespace cricket