Enable the -Wundef warning for clang

webrtc/system_wrappers/source/metrics_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/system_wrappers/include/metrics.h"
 #include "webrtc/system_wrappers/include/metrics_default.h"
 
 namespace webrtc {
 namespace {
 const int kSample = 22;
 
 void AddSparseSample(const std::string& name, int sample) {
   RTC_HISTOGRAM_COUNTS_SPARSE_100(name, sample);
@@ skipping 89 matching lines @@
 }
 
 #if RTC_DCHECK_IS_ON && GTEST_HAS_DEATH_TEST && !defined(WEBRTC_ANDROID)
 TEST_F(MetricsTest, RtcHistogram_FailsForNonConstantName) {
   AddSample("ConstantName1", kSample);
   EXPECT_DEATH(AddSample("NotConstantName1", kSample), "");
 }
 #endif
 
 }  // namespace webrtc