Enable the -Wundef warning for clang

webrtc/modules/video_coding/codecs/test/stats_unittest.cc

 /*
  *  Copyright (c) 2011 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/codecs/test/stats.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
 namespace test {
 
 class StatsTest : public testing::Test {
  protected:
   StatsTest() {}
 
   virtual ~StatsTest() {}
@@ skipping 25 matching lines @@
     FrameStatistic& frameStat = stats_->NewFrame(i);
     EXPECT_EQ(i, frameStat.frame_number);
   }
   EXPECT_EQ(nbr_of_frames, static_cast<int>(stats_->stats_.size()));
 
   stats_->PrintSummary();  // should not crash
 }
 
 }  // namespace test
 }  // namespace webrtc