Enable the -Wundef warning for clang

webrtc/modules/video_coding/utility/moving_average_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/modules/video_coding/utility/moving_average.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 TEST(MovingAverageTest, EmptyAverage) {
   webrtc::MovingAverage moving_average(1);
   EXPECT_EQ(0u, moving_average.size());
   EXPECT_FALSE(moving_average.GetAverage(0));
 }
 
 // Test single value.
 TEST(MovingAverageTest, OneElement) {
   webrtc::MovingAverage moving_average(1);
@@ skipping 29 matching lines @@
   for (int i = 1; i < 11; i++) {
     moving_average.AddSample(i);
   }
   EXPECT_EQ(*moving_average.GetAverage(), 5);
   moving_average.Reset();
   for (int i = 1; i < 2001; i++) {
     moving_average.AddSample(i);
   }
   EXPECT_EQ(*moving_average.GetAverage(), 1995);
 }