Enable the -Wundef warning for clang

webrtc/modules/video_coding/histogram_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/modules/video_coding/histogram.h"
 
 namespace webrtc {
 namespace video_coding {
 
 class TestHistogram : public ::testing::Test {
  protected:
   TestHistogram() : histogram_(5, 10) {}
   Histogram histogram_;
 };
@@ skipping 45 matching lines @@
   EXPECT_EQ(2ul, histogram_.InverseCdf(0.5f));
 
   histogram_.Add(20);
   EXPECT_EQ(10ul, histogram_.NumValues());
   EXPECT_EQ(2ul, histogram_.InverseCdf(0.5f));
   EXPECT_EQ(5ul, histogram_.InverseCdf(0.5000001f));
 }
 
 }  // namespace video_coding
 }  // namespace webrtc