Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/rtcp_packet/sli_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/rtcp_packet/sli.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/test/rtcp_packet_parser.h"
 
 using testing::ElementsAreArray;
 using testing::make_tuple;
 using webrtc::rtcp::Sli;
 
 namespace webrtc {
 namespace {
 constexpr uint32_t kSenderSsrc = 0x12345678;
 constexpr uint32_t kRemoteSsrc = 0x23456789;
@@ skipping 47 matching lines @@
   sli.AddPictureId(kPictureId, kFirstMb, kNumberOfMb);
 
   rtc::Buffer packet = sli.Build();
   packet[3]--;  // Decrease size by 1 word (4 bytes).
   packet.SetSize(packet.size() - 4);
 
   EXPECT_FALSE(test::ParseSinglePacket(packet, &sli));
 }
 
 }  // namespace webrtc