Enable the -Wundef warning for clang

webrtc/common_audio/channel_buffer_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/base/gtest.h"
 #include "webrtc/common_audio/channel_buffer.h"
 
 namespace webrtc {
 
 namespace {
 
 const size_t kNumFrames = 480u;
 const size_t kStereo = 2u;
 const size_t kMono = 1u;
 
@@ skipping 34 matching lines @@
   EXPECT_DEATH(chb.set_num_channels(kStereo), "num_channels");
 }
 
 TEST(IFChannelBufferTest, SetNumChannelsDeathTest) {
   IFChannelBuffer ifchb(kNumFrames, kMono);
   EXPECT_DEATH(ifchb.ibuf()->set_num_channels(kStereo), "num_channels");
 }
 #endif
 
 }  // namespace webrtc