Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/rtp_packet_history_unittest.cc

 /*
  *  Copyright (c) 2012 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/rtp_rtcp/source/rtp_packet_history.h"
 
 #include <memory>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_packet_to_send.h"
 #include "webrtc/system_wrappers/include/clock.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
 
 class RtpPacketHistoryTest : public ::testing::Test {
  protected:
   static constexpr uint16_t kSeqNum = 88;
@@ skipping 187 matching lines @@
 
   fake_clock_.AdvanceTimeMilliseconds(100);
 
   // Retransmit all packets currently in buffer.
   for (size_t i = 1; i < RtpPacketHistory::kMaxCapacity + 1; ++i) {
     EXPECT_TRUE(hist_.GetPacketAndSetSendTime(kSeqNum + i, 100, false));
   }
 }
 
 }  // namespace webrtc