Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/rtcp_packet/dlrr_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 "webrtc/modules/rtp_rtcp/source/rtcp_packet/dlrr.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/modules/rtp_rtcp/source/byte_io.h"
 
 using webrtc::rtcp::Dlrr;
 using webrtc::rtcp::ReceiveTimeInfo;
 
 namespace webrtc {
 namespace {
 const uint32_t kSsrc = 0x12345678;
 const uint32_t kLastRR = 0x23344556;
@@ skipping 68 matching lines @@
   dlrr.Create(buffer);
 
   // Parse.
   Dlrr parsed;
   uint16_t block_length = ByteReader<uint16_t>::ReadBigEndian(&buffer[2]);
   EXPECT_EQ(used_buffer_size, (block_length + 1) * 4u);
   EXPECT_TRUE(parsed.Parse(buffer, block_length));
   EXPECT_TRUE(parsed.sub_blocks().size() == Dlrr::kMaxNumberOfDlrrItems);
 }
 }  // namespace webrtc