Enable the -Wundef warning for clang

webrtc/modules/video_coding/percentile_filter_unittest.cc

 /*
  *  Copyright 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 <algorithm>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/modules/video_coding/percentile_filter.h"
 
 namespace webrtc {
 
 class PercentileFilterTest : public ::testing::TestWithParam<float> {
  public:
   PercentileFilterTest() : filter_(GetParam()) {
     // Make sure the tests are deterministic by seeding with a constant.
     srand(42);
@@ skipping 71 matching lines @@
     EXPECT_EQ(expected_value, filter_.GetPercentileValue());
 
     std::random_shuffle(zero_to_nine, zero_to_nine + 10);
     for (int64_t value : zero_to_nine)
       filter_.Insert(value);
     EXPECT_EQ(expected_value, filter_.GetPercentileValue());
   }
 }
 
 }  // namespace webrtc