Enable the -Wundef warning for clang

webrtc/modules/remote_bitrate_estimator/send_time_history_unittest.cc

 /*
  *  Copyright (c) 2015 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 <limits>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/remote_bitrate_estimator/include/send_time_history.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace webrtc {
 namespace test {
 
 static const int kDefaultHistoryLengthMs = 1000;
 
 class SendTimeHistoryTest : public ::testing::Test {
@@ skipping 204 matching lines @@
   EXPECT_TRUE(history_.GetInfo(&info2, true));
   EXPECT_EQ(packets[1], info2);
 
   PacketInfo info3(0, 0, packets[2].sequence_number, 0, 2);
   EXPECT_TRUE(history_.GetInfo(&info3, true));
   EXPECT_EQ(packets[2], info3);
 }
 
 }  // namespace test
 }  // namespace webrtc