Enable the -Wundef warning for clang

webrtc/video/video_decoder_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 "webrtc/video_decoder.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/checks.h"
 #include "webrtc/modules/video_coding/include/video_error_codes.h"
 
 namespace webrtc {
 
 class VideoDecoderSoftwareFallbackWrapperTest : public ::testing::Test {
  protected:
   VideoDecoderSoftwareFallbackWrapperTest()
       : fallback_wrapper_(kVideoCodecVP8, &fake_decoder_) {}
 
@@ skipping 139 matching lines @@
   EncodedImage encoded_image;
   fallback_wrapper_.Decode(encoded_image, false, nullptr, nullptr, -1);
   // Hard coded expected value since libvpx is the software implementation name
   // for VP8. Change accordingly if the underlying implementation does.
   EXPECT_STREQ("libvpx (fallback from: fake-decoder)",
                fallback_wrapper_.ImplementationName());
   fallback_wrapper_.Release();
 }
 
 }  // namespace webrtc