Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/packet_loss_stats_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/rtp_rtcp/source/packet_loss_stats.h"
 
 namespace webrtc {
 
 class PacketLossStatsTest : public ::testing::Test {
  protected:
   PacketLossStats stats_;
 };
 
 // Add a lost packet as every other packet, they should all count as single
@@ skipping 166 matching lines @@
     stats_.AddLostPacket((i + 4) & 0xFFFF);
     stats_.AddLostPacket((i + 1) & 0xFFFF);
     stats_.AddLostPacket((i + 2) & 0xFFFF);
   }
   EXPECT_EQ(100, stats_.GetSingleLossCount());
   EXPECT_EQ(200, stats_.GetMultipleLossEventCount());
   EXPECT_EQ(400, stats_.GetMultipleLossPacketCount());
 }
 
 }  // namespace webrtc