Enable the -Wundef warning for clang

webrtc/system_wrappers/source/metrics_default_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/system_wrappers/include/metrics.h"
 #include "webrtc/system_wrappers/include/metrics_default.h"
 
 namespace webrtc {
 
 namespace {
 const int kSample = 22;
 const char kName[] = "Name";
 
@@ skipping 123 matching lines @@
   metrics::GetAndReset(&histograms);
   EXPECT_EQ(1u, histograms.size());
   EXPECT_EQ(kName, histograms.begin()->second->name);
   EXPECT_EQ(1, histograms.begin()->second->min);
   EXPECT_EQ(100, histograms.begin()->second->max);
   EXPECT_EQ(50u, histograms.begin()->second->bucket_count);
   EXPECT_EQ(1u, histograms.begin()->second->samples.size());
 }
 
 }  // namespace webrtc