Enable the -Wundef warning for clang

webrtc/video/stats_counter_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 "webrtc/video/stats_counter.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace webrtc {
 namespace {
 const int kProcessIntervalMs = 2000;
 
 class StatsCounterObserverImpl : public StatsCounterObserver {
  public:
   StatsCounterObserverImpl() : num_calls_(0), last_sample_(-1) {}
@@ skipping 407 matching lines @@
   counter.ProcessAndGetStats();
   EXPECT_EQ(1, observer->num_calls_);
   // Make next interval pass, [6:1][24:1]
   clock_.AdvanceTimeMilliseconds(1);
   counter.ProcessAndGetStats();
   EXPECT_EQ(2, observer->num_calls_);
   EXPECT_EQ(24, observer->last_sample_);
 }
 
 }  // namespace webrtc