Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/rtp_packet_unittest.cc

 /*
  *  Copyright (c) 2016 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_received.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_packet_to_send.h"
 
-#include "testing/gmock/include/gmock/gmock.h"
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gmock.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/random.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_header_extensions.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_header_extension.h"
 
 using testing::ElementsAreArray;
 using testing::make_tuple;
 
 namespace webrtc {
 namespace {
 constexpr int8_t kPayloadType = 100;
@@ skipping 271 matching lines @@
   int32_t time_offset;
   EXPECT_FALSE(packet.GetExtension<TransmissionOffset>(&time_offset));
   packet.IdentifyExtensions(&extensions);
   EXPECT_TRUE(packet.GetExtension<TransmissionOffset>(&time_offset));
   EXPECT_EQ(kTimeOffset, time_offset);
   EXPECT_EQ(0u, packet.payload_size());
   EXPECT_EQ(0u, packet.padding_size());
 }
 
 }  // namespace webrtc