Enable the -Wundef warning for clang

webrtc/modules/pacing/bitrate_prober_unittest.cc

 /*
  *  Copyright (c) 2014 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 <limits>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/pacing/bitrate_prober.h"
 
 namespace webrtc {
 
 TEST(BitrateProberTest, VerifyStatesAndTimeBetweenProbes) {
   BitrateProber prober;
   EXPECT_FALSE(prober.IsProbing());
   int64_t now_ms = 0;
   EXPECT_EQ(-1, prober.TimeUntilNextProbe(now_ms));
 
@@ skipping 63 matching lines @@
 TEST(BitrateProberTest, DoesntInitializeProbingForSmallPackets) {
   BitrateProber prober;
   prober.SetEnabled(true);
   EXPECT_FALSE(prober.IsProbing());
 
   prober.OnIncomingPacket(100);
   EXPECT_FALSE(prober.IsProbing());
 }
 
 }  // namespace webrtc